The Cambridge Natural History Part 26

As with other Oreodonts, the molars are selenodont. The premaxillae are toothless--at least in adults, for two teeth are present in the {332} young. There are several species. _Agrioch.o.e.rus_, like _Oreodon_ and primitive Ungulates in general, had a long tail. The genus thus shows a mixture of ancient and specialised characters.

[Ill.u.s.tration]

FIG. 178.--Skeleton of _Agrioch.o.e.rus latifrons_. 1/8. (After Wortman.)

The most ancient form of Oreodont is _Protoreodon_. This is Eocene, and became extinct during that period. It had a complete dent.i.tion, open orbit, and no lachrymal fossa. The fore-feet were five-toed, the hind four-toed.

FAM. ANOPLOTHERIIDAE.--This family is entirely Eocene in point of time, and is unknown outside Europe. The dent.i.tion of the group is complete; the molars are seleno-bunodont, like those of the Anthracotheriidae. The bones of the carpus, tarsus, metacarpus and metatarsus are all free; the toes are four to two in number on each foot. The orbit is widely open behind. The tail is long, as in _Xiphodon_, etc. {333}

These general characters only just serve to differentiate the family; but they ill.u.s.trate its archaic character, in which it resembles the Xiphodontidae, and even more the Anthracotheriidae. A survey of some of the genera which have been a.s.signed to the family will bring out other features in the organisation of these very ancient Artiodactyles.

_Anoplotherium_ is so called on account of the fact that it is, like all ancient Artiodactyles, without horns or claws. Tusks it might have, but as a matter of fact has not. There are, as in Artiodactyles generally, nineteen dorso-lumbar vertebrae; the long tail has numerous chevrons. The shoulder blade has a well-marked acromion and a distinct coracoid process; it is wide proximally. The bones of the fore-arm and fore-leg are, as is usual in primitive Artiodactyles, separate.

In the skull the chief features, in addition to that mentioned in the definition of the family, are the large size of the paroccipital processes; there is no fossa lachrymalis or deficiency in the side of the face. The animal is three-toed, both in the fore- and hind-limbs. The second toe is nearly as large as the Artiodactyle third and fourth. There are tiny rudiments of the two remaining fingers. The hind-foot is also three-toed, and there is a trace of the hallux. The fingers are so widely separated and divergent from each other that it has been suggested that the animal had webbed feet and inhabited marshes, in which it swam by the aid of its long tail. The creature was the size of a Tapir.

Closely resembling _Anoplotherium_ are a number of other genera.

_Diplobune_ ( = _Hyracodontotherium_) was much like the last, but was a more delicately-formed animal. The fingers and toes (three of each) end in such sharply-pointed phalanges that claws seem to be almost suggested.

There are several species of this genus. _Dacrytherium_ differs by the presence of a lachrymal fossa.

_Dichobune_ has four-toed extremities, of which the lateral ones are more slender and shorter than the two middle ones. As in other Anoplotheriidae, the anterior premolars are furnished with a sharp cutting edge.

ORDER V. SIRENIA.

Aquatic Mammalia, with but few scattered hairs; hind-limbs absent; fore-limbs paddle-shaped; tail flattened, and either {334} Whale-like or rhomboidal to circular in form. Nostrils on upper surface of not specially-elongated snout. Clavicles are absent. The scapula has the normal mammalian form, with a well-developed and roughly median spine. The bones of the arm and hand articulate together, as in land animals; the phalanges show at most traces of increase in number above the normal. Pelvis represented by a vestige, more highly developed in some fossil than in recent forms. Stomach complex, consisting of several chambers. Lungs simple and not lobulated. Diaphragm oblique and very muscular. Brain peculiar in form and but slightly convoluted. Testes abdominal. Teats two, and pectoral in position. Placenta non-deciduous and zonary.[222]

This limited group consists of purely aquatic forms, which are both marine and fresh-water in their proclivities. They have been placed in the immediate vicinity of the Whales; but it is now believed by most zoologists that the likenesses which they undoubtedly show to the Cetacea are of an adaptive kind and related to their similar mode of life. The group is a readily-definable one. Externally they are marked by their dark coloration, somewhat Whale-like though of clumsier build, and by the total absence of external ears and hind-limbs; the latter are, however, as will be pointed out shortly, marked by certain rudimentary bones. There is a flattened tail, which in the Dugong and _Rhytina_ is precisely like that of a Whale.

It is interesting to note that the former genus, whose tail is, judging it at least by the standard of the Whales, more completely modified for the aquatic life, should also show other features which indicate their longer life as marine creatures. For the flippers are more Whale-like in that the fore-arm is completely enclosed within the body, or nearly so, and the nostrils have a more decidedly superior position than in the Manatee. The fore-limbs of this group, as may be inferred from what has just been said, are flipper-like; but, contrary to what we find in Whales, the phalanges do not as a rule show any traces of multiplication, so characteristic a feature of the Cetacean hand, and the individual bones are connected by well-formed joints. Beneath the thick skin, which is spa.r.s.ely provided with stout hairs in the Dugong, is a layer of blubber. Dr. Murie has called attention to the fact that this layer in the {335} Manatee[223] differs from the blubber of the Whale in that there is no free oil anywhere.[224]

The skeleton of the Sirenia is strong and ma.s.sive, thus contrasting with the loosely-textured bones of the Cetacea. The cervical vertebrae are, as a rule, free, but the second and third are fused in _Manatus_ and the extinct _Halitherium_. It is noteworthy that in _Rhytina_ the cervical vertebrae have the exceedingly thin centra that characterises the neck vertebrae in Whales. The ribs are most of them firmly articulated by two heads. The breastbone is generally reduced, as in Whales; and but few ribs are attached thereto. The vertebrae, moreover, are well locked together by zygapophyses, and not loosely attached as in Whales.

The shoulder blade is long and narrow, and not unlike that of the Seals. It is totally unlike the peculiarly-modified scapula of the Whale tribe. But, as in the latter, there are no clavicles.

The hind-limbs are only represented by the pelvis; and this is a rudimentary structure, varying, however, in the degree of its degeneration.

That of the extinct _Halitherium_ recalls the pelvis of the Rorqual. There is a single triradiate bone with an acetabular cavity for the rudiment of the femur in the centre; it suggests that here the three normal elements of the pelvis have become fused into a single bone. In the Dugong there are two small bones on each side.

The Manatees (_Manatus_)[225] are found in the fresh-waters and along the Atlantic coasts of South America and Africa. It appears that there are four species, of which one only is African, the others American. Report a.s.serts the former occurrence of this genus on the sh.o.r.es of St. Helena.

The Manatee is provided with only six cervical vertebrae, a fact which distinguishes it from the other existing genera of its group. A remarkable feature which it exhibits is the large number of molar teeth. These apparently go on increasing indefinitely during its life, the suggestion being that they are worn away by the nature of the food--algae with much sand intermixed. As many as twenty molar teeth have been counted in one half of the jaw, and there is no reason to forbid the a.s.sumption that they {336} may get still more numerous. This large number of grinding teeth is obviously suggestive of the Whales, with which the Sirenia are believed by some to be allied. It is at least a remarkable coincidence that these two aquatic groups of mammals should both have a.s.sumed the same indefinite tooth formula. It is correct to say a.s.sumed, since extinct forms of Manatees, such as _Halitherium_ and _Prorastoma_, have not a continuous succession of molars. The brain of the Manatee is, contrary to the usual arrangement among aquatic mammals, smooth, and only marked by one or two fissures.

The Manatee[226] is black in colour, its thick skin being wrinkled. The animal is a.s.sisted in feeding by a curious mechanism of the upper lip; this is split in two, and the two halves, which are furnished with strong bristles, can play upon each other like the points of a pair of forceps.

The flippers are furnished with nails, save in _M. inunguis_, but in the nailed forms it is not every finger which is thus armed.

[Ill.u.s.tration]

FIG. 179.--Skeleton of Dugong. _Halicore australis._ (After de Blainville.)

_Halicore_,[227] the Dugong, is an entirely Oriental and Australian {337} form; there appears to be but a single species, though more than one name has been given to supposed distinct species. As already mentioned, it differs from the Manatee in the possession of a Whale-like tail; the nostrils, too, are more upon the upper surface of the head, and there are no nails upon the flipper. The peculiar cleft lip of the Manatee is not so well developed in the Dugong, but there are traces of it; and in the foetus the likeness to the Manatee in this respect is very striking. It would thus appear that _Halicore_ is a stage in advance upon _Manatus_; that the remarkable mechanism of the lip of the latter has been possessed, but has been lost, by the Dugong. The skull of the Dugong is distinguished by the stout premaxillary bones, which bear a tusk in the male. In the female the tooth is there, but is lodged within the bone. This incisor has a milk forerunner. The back teeth of the Dugong (there are no canines) are few in number (four or five, even six), thus showing a gradual reduction when compared with _Manatus_; and this culminates in the toothless _Rhytina_. It is also interesting to notice that in the ma.s.sive lower jaw there are traces of an incisor. Were this to be developed into a tusk, the jaw would present a curious resemblance to that of _Dinotherium_.

The Dugong, _H. dugong_, has the reputation of being the original of the mermaid legends, since the young is held to the pectorally-situated breast with one flipper. "But it should be remembered," justly observes Dr.

Blanford, "that stories of beings half man or woman, half fish, are as common in temperate as in tropical seas, and that some of them are more ancient than any European knowledge of the Dugong."

EXTINCT SIRENIANS.--The earliest genus that can be with certainty referred to this order is the Oligocene _Prorastoma_. This genus, though offering no particular skull-characters that a.s.sist in the determination of the much-debated affinities of the Sirenia, shows a remarkable condition of the teeth that may afford a clue. The species _P. veronense_, recently described by Mr. Lydekker,[228] is founded upon a fragment of the skull which contains two teeth apparently representing the third and fourth upper milk molars. The interest attaching to these teeth lies in the fact that they clearly exhibit the buno-selenodont condition characteristic of certain early Artiodactyles, e.g. _Merycopotamus_.

_Halitherium_ is a later genus, which is known by the nearly {338} complete skeleton. The skull is like that of other Sirenia, with the down-turned premaxillary region. But the nasal bones, lost, or at least rudimentary, in recent forms, are well developed; the likeness of ancient to living forms in this respect being exactly paralleled by the Zeuglodonts, when compared with recent Whales. The vertebral centra exhibit distinct epiphyses, which have disappeared in living Sirenians. The cervical vertebrae are seven, of which the second and third are occasionally fused. There are nineteen pairs of ribs, and there are three lumbar vertebrae. The sternum consists of three separate pieces. There is a rudimentary femur.

The recently-extinct Steller's Sea-cow, belonging to the genus _Rhytina_, was a huge beast, seen in the flesh up to nearly the end of the last century. It frequented the sh.o.r.es of Bering's Straits. Its remains occur in the peat on the sh.o.r.es of those seas. It reaches a length of some 20 to 30 feet. The external characters were much like those of other recent Sirenians. The nostrils were above the fore part of the snout, the latter being truncated and obtuse. The tail was of the Cetacean pattern, and thus like that of _Halicore_. The head of this Sirenian was small, and the teeth had entirely vanished save for the apparent existence as transitory structures of two small incisors in the upper jaw. The absence of teeth was compensated by the presence of a h.o.r.n.y palate for the trituration of the sea-weeds which const.i.tuted the food of Steller's Sea-cow. The fore-limbs seem to have possessed no nails, but were covered at the extremity with short, bristly hairs, no doubt serving the purpose of keeping the animal moored in safety to the slippery beds of Fucus upon which it browsed.

There are nineteen pairs of ribs. The vertebrae of the cervical region are the customary seven, and the centra are thin and plate-like as in the Cetacea, the animal being thus short-necked like those marine creatures.

{339}

CHAPTER XII

CETACEA--WHALES AND DOLPHINS

ORDER VI. CETACEA.[229]

Aquatic Mammalia of fish-like form; tail expanded into horizontal flukes; a fatty dorsal "fin" present in most species; anterior limbs converted into fin-like paddles; posterior limbs only represented by skeletal rudiments.

Hairy covering reduced to a few isolated hairs in the neighbourhood of the muzzle. Nostrils represented by the single or double blow-hole, nearly always situated far back upon the skull. Bones of loose texture and much impregnated with oil. The skull has a greatly-developed facial portion; supra-occipital bones meeting the frontal by overgrowing, or growing in between the parietals; bones surrounding the organ of hearing loosely attached to the skull, the tympanics of peculiar cowrie-sh.e.l.l form.

Coronoid process of mandible absent, or very feebly developed. Teeth, when present, few or numerous, always of simple conical form, with at most traces of additional cusps (_Inia_); if absent their place taken by whalebone. Cervical vertebrae of short antero-posterior diameter, often more or less completely welded together into a single ma.s.s. Articulations between dorsal and other vertebrae feeble. Scapula peculiarly flattened; acromion strongly developed as a rule, but arising from a slightly-marked spine; coracoid process generally strongly developed. Phalanges of digits always more numerous than in other mammals. Clavicles absent. Stomach complex, consisting of at least four and often more chambers. Lungs simple and non-lobulated. Diaphragm obliquely set and very {340} muscular. Brain much expanded transversely and well convoluted. Testes abdominal. Teats two, inguinal in position. Placenta diffuse and non-deciduate.

The Whales and Dolphins, which const.i.tute this order, form an a.s.semblage which is easily characterised by reason of the fact that their affinities to other groups of Mammalia are so doubtful that they furnish matter rather for speculation than for authoritative statement. Some hold that they resemble in certain points the Ungulata; while others again see in them the culminating term of a series which commences with such a form as the Otter, and of which the Seals and Sea-lions are intermediate stages. A third opinion is that the Whales have arisen from some low mammalian stock, too primitive to be a.s.signed to any existing order of mammals. Palaeontology, as will be seen later, throws no light whatever upon their origin. This matter has already been referred to (see p. 120) in considering the position of the Cetacea.

The Whales include the most gigantic of all the orders of vertebrated animals. No creature living or extinct is so large as the Sibbald's Rorqual, which attains to a length of some 85 feet, or perhaps even rather more. On the other hand we have what are by comparison minute forms. Apart from the possibly problematical _Delphinus minutus_, stated to be only 2 feet in length, we have as a minimum 3 or 4 feet. The size of the Cetacea has been subjected to much exaggeration. The first duty of a Whale, observed the late Sir William Flower, is to be large; and Natural Historians, in the recent as well as in the remote past, have not hesitated to put very round numbers upon the dimensions of the larger members of the order. We may perhaps pa.s.s over Pliny's "fish called balaena or whirlpool, which is so long and broad as to take up more in length and breadth than two acres of ground," and a number of a.n.a.logous exaggerations, which gradually dwindled down to the dimensions just stated of the great Rorqual.

M. Pouchet has made the ingenious suggestion that the statements of the ancients may have been nearer the truth than observations of to-day would have us believe; he pointed out justly that in former times Whales were not so relentlessly pursued as during the last century; the inference being that they may have lived to a greater age, and attained a more colossal bulk. The more modern exaggerations in the {341} dimensions of the bigger Whales are probably due to the fact that measurements have been taken, not in a straight line from snout to tail, but along the bulging sides of the Cetacean, rendered even more convex than in nature by decomposition, and by pressure due to the immense tonnage of the creature.

The Cetacea are the most perfectly aquatic of all mammals; they never leave the waters which they inhabit. It is true that legends have represented them as pasturing upon the sh.o.r.e--Aelian spoke of Dolphins basking in the sun's rays upon the sand; and the "Devil Fish" of California, _Rhachianectes_ (see p. 357) has given rise to improbable stories--but they are apparently only legends. Indeed a stranded Whale cannot live long, for it is unable to breathe, the comparatively feeble breast being crushed by its own weight. In accordance with the purely aquatic habit, we find a modification of the outward form of the body (and as we shall see later of many of the internal organs), which renders the Cetacea externally unlike all other mammals. The form is fish-like, the fore-limbs are paddles, the tail is expanded into two horizontal flukes, which serve to propel the creature through the water.

[Ill.u.s.tration]

FIG. 180.--Killer. _Orca gladiator._ 1/40 (After True.)

The skin is smooth and shiny, so smooth and so shiny that it has often been compared to coach leather. But nevertheless they are not entirely without that most essential character of the cla.s.s Mammalia, a coating of hair. The hairy covering is, however, reduced to the very smallest proportions; it is represented {342} by a few hairs only--so few that they can be counted with ease--in the neighbourhood of the muzzle. These hairs are not present in all Whales; they are absent, for example, in the White Whale or Beluga.

When present they are not furnished either with sebaceous glands or with muscular fibres, which are such universal concomitants of the hair follicles in the Mammalia generally. This appears to be conclusive evidence that the hairs, few as they are, are still undergoing degeneration. The need for a furry coat is removed by the presence of a thick coating of fat immediately underlying the skin. This is known as the blubber, and is the main incentive to the pursuit of Whales. It must not, however, be a.s.sumed without further argument that the hair is absent because its place is taken, as a mechanism for retaining the heat, by the blubber; for the Seal tribe possess both fur and blubber. Another conceivable explanation is quite at variance with such a view of economy. It may be noticed that among Ungulates there is a tendency to lose hair, particularly among more or less aquatic forms. Thus the Hippopotamus is almost naked (as is indeed the Walrus); the Rhinoceros, too, often a frequenter of marshy soil, is almost as denuded as is the Hippopotamus. It is not, however, settled that the Whales have anything to do with the Ungulata; otherwise an additional argument might be used, that is, the secular loss of hair in some members of this group. The Hairy Rhinoceros, _Rh. tichorhinus_, was, as its name denotes, a hairy beast; the Mammoth was equally so. The descendants, or at least the modern representatives of both these creatures, are but scantily clad with hairs.

A final reason for the naked character of the skin in existing Cetacea is closely connected with a feature in the organisation of three or four living species which must first be described.

Some years ago the late Dr. J. E. Gray of the British Museum described from the sea, off Margate, what he considered to be a new species of Porpoise, characterised by the presence on the dorsal fin of a row of stony tubercles. As a matter of fact it was subsequently shown that the Common Porpoise has the same structures, so that there was no need for a Margate species, _Phocaena tuberculifera_. Moreover, in the Indian _Neomeris_, a close ally of the Porpoise, a more abundant calcified covering of scales exists along the whole back of the animal. These plates, {343} it has been discovered, are larger in the foetus, a fact which naturally points to their being an inheritance from the past, now undergoing retrogressive changes. Such a way of looking upon the facts is confirmed by the finding, many years ago, by the naturalist and physiologist Johannes Muller, of bony plates in connexion with the remains of a Zeuglodont Cetacean. It looks, therefore, very much as if the Eocene ancestors of the modern Cetacea had a skin studded with bony plates, as have the armadillos. This being the case, the disappearance of hair is not surprising. The room would be taken up by the calcified plates, and when the latter disappeared, as they have in the vast majority of existing Whales, the naked skin alone would be left.

Whales possess no externally-visible hind-limbs; rudiments of these appendages are present, which will be dealt with under the description of the princ.i.p.al features of the skeleton. But it has been discovered that in the Porpoise, external vestiges of hind-limbs do appear in the foetus, a fact which, be it observed, does away with the old view that the flukes of the Whale are the last term in the series of vanishing hind-limbs, of which the Seals, with their hind-limbs and tail bound up together, offer an intermediate step.

The tail is fish-like in form, but the flukes are horizontal instead of vertical as in fishes and _Ichthyosaurus_. This arrangement is no doubt a.s.sociated with the need for rapid return to the surface waters after a prolonged immersion in search of food. A downward stroke, such as is given by the powerful and large tail flukes, would naturally bring about this result rapidly. The tail, moreover, is under all circ.u.mstances the swimming organ. Its motion has been stated to be slightly rotatory, like that of a screw, and it is the case that the two flukes are often alternate in shape like the f.l.a.n.g.es of a screw; one being convex upwards, the other convex downwards.

The fore-limbs are in the form of paddles, but they do not apparently serve as organs of locomotion so much as balancers. When a Whale is killed, it falls over on to one side, the office of the flippers being to maintain the proper position. It is believed, however, from the fact that the embryo often shows a relatively larger pectoral fin than that of the adult--the difference being due to a reduction in the adult of the number of phalanges--that the fin was once an organ of progression. {344}