The Cambridge Natural History Part 37

_Monachus_ is a northern genus inhabiting the Mediterranean and the Atlantic in the vicinity of Madeira and the Canary Islands. It has rudimentary nails upon both pairs of feet. The first and fifth toe of the hind-feet are longer than the others. As with the preceding genera, the incisors are two in each jaw. The species are _M. albiventer_, the Monk-Seal, and _M. tropicalis_, the Jamaica Seal.

Allied to _Cystophora_ is the genus _Macrorhinus_, with (possibly) two species, of which one is Antarctic, the other frequents or frequented the coast of California. The incisors are two in the upper jaw, and but one in the lower. The premolars are four and the molar one; all the teeth are small and simple, but {454} have long roots. The nose of the male has a dilatable proboscis. The southern Elephant Seal is _M. leoninus_, and reaches a length of some 20 feet. It occurs on the sh.o.r.es of Kerguelen and some other more or less remote islands. Its habits have been studied and described by several observers, beginning with Anson in the last century.

The late Professor Moseley gave a good account of this marine monster in his book on the voyage of the "Challenger." When the animal is enraged, the end of the snout is dilated; but when this happens there is no long and hanging proboscis such as has sometimes been described. The inflation affects the skin on the top of the snout, which thus rises rather upwards during inflation. The inflated region, according to Mr. Vallentin, quoted by Mr. J. T. Cunningham, is about 1 foot long in an individual of 17 feet.

It has been stated that this proboscis is a temporary structure, only appearing in the breeding season; but recent observations have shown that this statement is inaccurate; it persists all the year round. The males fight greatly during the breeding season, and produce a roar which has been compared to the "noise made by a man when gargling." The females and the young males bellow like a bull. The males fight of course with their teeth, literally falling upon one another with their whole weight. Mr. Cunningham thinks that the use of the proboscis is to protect the nose from injury; or that it may be merely the result of "emotional excitement." In any case the Bladder-nosed Seal, _Cystophora_, is undoubtedly protected from injury by the possession of a corresponding hood. The nose is the most vulnerable place, and the existence of this hood would stave off the effects of a blow in that region. Moseley, however, has said of _Macrorhinus_ that it cannot be stunned by blows on the nose as other Seals can; but he attributes this, not to the dilated snout, but to the bony crest on the skull, and to the strength of the bones about the nose. This Seal crawls with difficulty on the land, and as the animals move "the vast body trembles like a great bag of jelly, owing to the ma.s.s of blubber by which the whole animal is invested, and which is as thick as it is in a whale."[324] When lying on the sh.o.r.e, these animals sc.r.a.pe sand and throw it over themselves, apparently to prevent themselves from being {455} incommoded by the direct rays of the sun, to the effects of which they are very susceptible. The Elephant Seal is mild and inoffensive, unless enraged, and, of course, during the breeding season.

ORDER VIII. CREODONTA.

This entirely extinct group of Mammalia may be thus characterised:--Small to large carnivorous mammals, with skull on the whole like that of the Carnivora and with trenchant teeth; digits with unguiculate phalanges; tail long; extremities usually with five, sometimes with four digits. In the carpus a centrale is present, and the scaphoid and lunar are separate.

Interlocking of posterior dorsal and lumbar zygapophyses very perfect.

Brain small but convoluted.

This group, which corresponds with the CARNIVORA PRIMIGENIA of Mr.

Lydekker, is not easy to separate absolutely from the existing and more especially from some of the extinct members of the CARNIVORA VERA. They also come exceedingly near the Condylarthra, the presumed ancestors of the Ungulata, and like them begin in the earliest Tertiary deposits. Their likeness to the carnivorous Marsupials has also been insisted upon; but it would seem that the succession of teeth in the Creodonta is typically Eutherian.

The characteristics of the group may be exemplified by an account of the genus _Hyaenodon_, after which some of the more important deviations in structure shown by other genera will be referred to.

_Hyaenodon_ is both American and European, and ranges through the Eocene and the Upper Miocene. It is a much-specialised Creodont, and therefore exhibits well the distinctive characters of the group. About a dozen species have been described. One of the best-known is the American _H.

cruentus_, and the following description refers to it. The back part of the skull is low and broad, and is compared by Professor Scott (who has described this and other species) as being "somewhat like that of an opossum."[325] The whole skull is {456} long, and the top has a great sagittal crest. The paroccipital processes are short and are closely applied to the mastoid processes. The mesethmoid is larger than in the carnivorous Marsupials, and the frontals are very large. The palate has a peculiar structure; in most species the hinder ends of the palatines are separated by a narrow fissure which broadens gradually, thus forming the posterior nares. In _H. leptocephalus_ the posterior nares are brought very far back by the meeting of the alisphenoids. The presphenoid, contrary to what we find in the Dog, for example, is chiefly concealed by the vomer, which covers it. The mandible has a long and strong symphysis, and its angle is not inflected. The fore-limb is described as being "weak when compared with the modern Carnivora." The scaphoid and lunar are separate, and there is a centrale. The teeth present us with nearly the typical formula. There is only one molar missing in the upper jaw. The canines are enlarged. It has been suggested from a consideration of its palate that _Hyaenodon_ was a semiaquatic animal; the deep cleaving at the extremities of the phalanges seems to point in the same direction, since they resemble in this the genus _Patriofelis_, which there are other reasons to regard as aquatic. This latter genus has a fore-limb which is very like that of the Pinnipedia, the digits are much spread out, and would seem to have supported a kind of paddle. In any case it certainly fed upon aquatic tortoises, for their remains have been found in its coprolites. The name _Limnofelis_, also applied to what appear to have been members of this genus, is suggestive of their habits. _Patriofelis_, at least one species, seems to have been of about the size of a Lion.

_Mesonyx_ has a brain case which is actually smaller than that of the Marsupial _Thylacinus_. The lachrymal bone is very large, and extends a little way over the face, as is also the case with _Hyaenodon_; this condition is also found in Insectivora and in _Thylacinus_. The axis vertebra has a curiously-shaped spine, which is very different from the hatchet-shaped process of that vertebra usual in the Carnivora, but is not unlike what exists in the Arctoid genera _Meles_ and _Mydaus_. The limbs show much disparity in length, and seem to argue a much-arched back when the creature progressed. The carpus is stated to be strikingly like that of the Insectivora. There is as in other Creodonts a separation between the scaphoid and lunar; {457} the centrale appears to be present. The pelvis "is most like that of the bear," the metacarpals and the tibia, and some other bones, resemble those of the Hyaena. In fact this animal shows those combined characters which are common in archaic forms.

{458}

CHAPTER XV

RODENTIA--TILLODONTIA

ORDER IX. RODENTIA[326]

Small to moderately large animals, furry, sometimes with spines. Toes with nails of a claw-like character, or sometimes approaching hoofs. Usually plantigrade, and only occasionally and partly carnivorous. Canine teeth absent; incisors long and strong, growing from persistent pulps, and with enamel only or chiefly on the anterior face, producing a chisel-shaped edge; molars few (two to six), separated from the incisors by a wide diastema. Caec.u.m (nearly always present) very large, and often complicated in structure. Brain, if not smooth, with few furrows, the hemispheres not overlapping the cerebellum. Surface of skull rather flat; orbits not separated from temporal fossae; malar bone in middle of zygomatic arch; palate very narrow, with elongated incisive foramina; articular surface for lower jaw antero-posteriorly elongated. Clavicles generally present. Testes generally abdominal. Placenta deciduate, and discoidal in form.

The Rodents are a very large a.s.semblage of usually small, sometimes quite minute, creatures, embracing an enormous number of living generic types.

They are distributed all over the world, including the Australian region, and, being small and often nocturnal, and by no means particular in their diet, have managed to thrive and multiply to a greater extent than any other group of living mammals. They are chiefly terrestrial creatures, and often burrow or live in ready-made burrows. {459} Some, however, such as the Voles, are aquatic; others, _e.g._ the Squirrels, are arboreal, and there are "flying" Rodents exemplified by the genus _Anomalurus_. Their range of habitat is in fact as wide as that of any other Order of mammals, and wider than that of most.

[Ill.u.s.tration]

FIG. 231.--Side view of skull of Cape Jumping Hare (_Pedetes caffer_).

3/5. _AS_, Alisphenoid; _Ex.O_, exoccipital; _Fr_, frontal; _L_, lachrymal; _Ma_, malar; _Mx_, maxilla; _Na_, nasal; _OS_, orbito-sphenoid; _Pa_, parietal; _Per_ points to the large supratympanic or mastoid bulla; _PMx_, premaxilla; _Sq_, squamosal; _Ty_, tympanic. (From Flower's _Osteology_.)

The most distinct anatomical characteristic of the Rodents concerns the teeth. They are without exception entirely deprived of canines. Thus there is a long diastema between the incisors and the molars. Another peculiarity is, that in many cases the dent.i.tion is absolutely monophyodont. In such forms as the Muridae there seems to be no milk dent.i.tion at all. In that family there are only three molars; but in other types where there are four, five, or six molars, the first one, two, or three, as the case may be, have milk predecessors, and may thus be termed premolars. This has been definitely proved to be the case in the common Rabbit, which has the unusually large number of six grinding teeth in each half of the upper jaw when adult. The first three of these have milk forerunners. On the other hand the existence of four molars does not apparently always argue that the first is a premolar; for Sir W. Flower found that in _Hydroch.o.e.rus_,[327]

none of the teeth had any forerunners, at any rate so far as could be detected from the examination of a very young animal. The Rabbit appears to be also exceptional, in that the second incisor of the upper jaw and the incisor of the lower jaw have milk forerunners. In any case the tendency towards monophyodontism is peculiarly well-marked in this group of mammals.

The incisors of Rodents are as a rule in each jaw a single pair of long and strong teeth, which grow from persistent pulps, and {460} grow to a very great length, extending back within the jaw to near the hinder part of the skull. These teeth are reinforced in the upper jaw by a small second pair in the Lagomorpha only. The incisors are chisel-shaped, and often brown or yellow upon the outer face, as is the case also with some Insectivores.

This peculiar shape, and their strength, renders them especially capable of the gnawing action which characterises the Rodents. It has been pointed out that where the incisors are wider than thick, the gnawing powers are feebly developed; and that on the contrary, where these teeth are thicker than wide, the animals are good gnawers. The incisors have often an anterior groove, or it may be grooves.

[Ill.u.s.tration]

FIG. 232.--Molar teeth of Rodents. A, of Capybara (_Hydroch.o.e.rus_); B, of Squirrel (_Sciurus_); C, of _Ctenodactylus_. (After Tullberg.)

The cheek teeth vary in number from two (_Hydromys_) to six (Rabbit) on each side of the two jaws. Four is the prevailing number outside the large division of the Rat-like Rodents. They are often set at an angle to the horizontal plane of the jaw, looking outwards and obliquely to its longitudinal axis; the individual teeth too are not unfrequently bowed in form, reminding us of those of _Toxodon_. This of course only occurs in those genera which have hypselodont teeth. The pattern of the teeth varies much, and the different forms recall the teeth of more than one other group of mammals. They are either bunodont or lophodont. In many cases the tooth is encircled with a ridge of enamel, which is either almost simple or has a more complicated contour; such teeth are by no means unsuggestive of the Toxodonts. Some of the lophodont molars are by no means unlike those of the Proboscidea. In _Sciurus vulgaris_ the {461} encircling ridge is broken up into tubercles, which gives to the tooth a striking likeness to those of _Ornithorhynchus_. Other genera have teeth like those of many Ungulates. It has been shown by Sir J. Tomes[328] that the minute structure of the enamel differs in different groups of Rodents.

The skull shows certain primitive characters. In the first place there is no distinction between the orbital and the temporal fossa.[329] The sutures between the bones retain their distinctness for very long. Other characteristic features are the following:--The nasals are large, and so are the paroccipital processes. The palate in front of the molars is not distinct from the sides of the skull, its edge gradually becoming rounded off above. It is also very narrow. The premaxillae are large in relation to the great incisors. There is often a very much enlarged infra-orbital foramen through which pa.s.ses a part of the ma.s.seter muscle. The jugal bone lies in the middle of the zygomatic arch, which is complete and enormously enlarged in the Spotted Cavy (_Coelogenys paca_). As in many Marsupials, the jugal bone sometimes extends backwards to the glenoid cavity, where the lower jaw articulates. It is usually said with an absolute want of accuracy that the cerebral hemispheres of the Rodents are smooth and without convolutions. This error has been repeated again and again in text-books.

As a matter of fact the cerebral hemispheres of many forms are quite well convoluted,[330] the degree of furrowing corresponding, as in so many groups of mammals, with the size of the animal. This at any rate is generally true, though the large Beaver with its scant convolutions is an exception. The smaller forms, such as _Mus_, _Sciurus_, _Dipus_, and _Cricetus_ are quite smooth-brained. The best furrowed brain of any Rodent which has been examined is that of the huge _Hydroch.o.e.rus_. The Sylvian fissure is very generally not p.r.o.nounced; but is particularly well-marked in _Lagostomus_. In all, or in most, Rodents the hemispheres are separated by an interval from the cerebellum, the optic lobes being visible between the two.

The mouth cavity of this group of mammals is divided into two chambers by a hairy ingrowth behind the incisors; this arrangement is useful for animals which use their strong incisors {462} as gnawing and excavating tools as well as for the purposes of alimentation; for it allows of substances being gnawed away without the products of the chisel-like action being taken into the hinder cavity of the mouth. The Rodents have for the most part a simple stomach of normal form; but in a few this is complicated by a marked constriction, which divides the cardiac from the pyloric portions. The Hamster, for example, is thus characterised. In all the members of the order, with the exception of the Dormice and some allied forms, the caec.u.m is large and often sacculated. In some forms (e.g. _Arvicola_, _Myodes_, _Cuniculus_) the large intestine is coiled upon itself in a spiral way--a state of affairs strongly suggestive of Ruminants.

The Rodents are primarily divisible into two great groups, the Simplicidentata and the Duplicidentata, characterised mainly by the upper incisor teeth. In the former there is but one pair of these teeth; in the latter a second smaller pair lie behind the former.

SUB-ORDER 1. SIMPLICIDENTATA.

SECTION 1. SCIUROMORPHA.

The Anomaluri are separated by Thomas and others from this section as an equal section, while by Tullberg they are grouped with _Pedetes_.

FAM. 1. ANOMALURIDAE.--The genus _Anomalurus_ suggests at first sight the Flying Squirrels of Asia, _Pteromys_. It is, however, an entirely African genus, and is to be distinguished from the Asiatic Rodents by a series of scales at the root of the tail, imbricated, keeled, and forming possibly a "climbing organ." This character serves also to distinguish the present genus from _Sciuropterus_. The cartilage, moreover, which supports the patagium springs from the elbow. There are four molars in each half of each jaw. The eyes and ears are large. There are five fingers and toes, but the thumb is small, though provided with a nail. The sternum has seven joints, and nine ribs reach it. The clavicle is strong. Huet, who has recently monographed the genus,[331] allows six species. The species vary in size.

{463}

_Anomalurus peli_ appears, according to Mr. W. H. Adams,[332] to be a common species on the Gold Coast; it is coloured black and white, but in spite of the warning which this colour should convey, is considered by the perhaps rather omnivorous native as "the greatest delicacy." The animal is nocturnal, but affects only bright moonlight nights. Their "flying"

consists of a jump from a high branch to a lower one, after which they reascend the tree to a point of vantage for another jump. They are said to feed upon nuts; but Tullberg only found the remains of leaves in the stomach.

_Idiurus_ is a lately-described genus allied to _Anomalurus_. There are at any rate two species, _I. zenkeri_ and _I. macrotis_. The thumb is more reduced than in _Anomalurus_, and the fibula, contrary to what is found in that genus and in most Sciuromorphs, is fused with the tibia below.

A third genus, very recently described and allied to both the foregoing, is _Aethurus_. It is a native of the French Congo,[333] and differs by the absence of flying membranes. It has, however, the pad of large scales.

There is but one species, _A. glirinus_. It has a black bushy tail. The pos...o...b..tal processes of the frontals are totally wanting--there are not even the traces to be seen in _Anomalurus_. The thumb has vanished. If we are to compare _Anomalurus_ with the Squirrels then, thinks Mr. de Winton, the present genus is probably diurnal by reason of the want of flying membranes.

FAM. 2. SCIURIDAE.--The Squirrels, genus _Sciurus_, are world-wide in range, the Australian region and the island of Madagascar being alone excepted.

The eyes and the ears are large; the tail is of course long and bushy. The fore-feet have an inconspicuous thumb; the hind-feet have four toes. The soles of the fore-feet are naked or furry, those of the hind-feet are hairy. There are twelve or thirteen dorsal vertebrae, and in correspondence seven or six lumbars. The caudal vertebrae may be as many as twenty-five.

In the skull the frontals are broad, and there are long {464} pos...o...b..tal processes. The infra-orbital foramen is, as a rule, not large, but is increased in size in a few forms. The number of separate pieces of bone in the sternum is five. The molars of the upper jaw are five, but the first is very small and soon drops out.

The Squirrels are often rather brilliantly coloured. The Chinese _Sc.

castaneiventris_ has grey fur with a rich chestnut-coloured under surface.

The Malabar Squirrel, _Sc. maximus_, as its name implies, a large animal, has a deep reddish or chestnut-coloured fur above, which becomes yellow below. The "Common Squirrel," "the lytill squerell full of besynesse,"

which is the Squirrel of this country, is brownish red on the upper parts and white below. It ranges from this country as far east as j.a.pan. Like many other Rodents the Squirrel likes animal food and will eat both eggs and young birds. "Camel's hair" brushes are made from this animal. The genus _Tamias_, almost exclusively North American in range, is included by Dr. Forsyth Major[334] in this genus, which then consists of considerably over one hundred species.

The Ethiopian Ground Squirrels, genus _Xerus_, have a more elongated skull than _Sciurus_, and the pos...o...b..tal processes are shorter. The feet are not hairy.

_Nannosciurus_ forms a perfectly distinct genus of Squirrels. These "Pygmy Squirrels" differ in possessing a very elongated "face" and in the very broad frontal region. The teeth are unlike those of _Sciurus_ in certain features, and have been especially compared by Forsyth Major to those of the Dormice. Four species of this genus are Malayan; one is West African.

The Bornean _Rheithrosciurus macrotis_ is the only species of its genus.

The genus may be distinguished by the exceedingly brachyodont molars, this feature being more marked in this genus than in all other Squirrels. It is called the "Groove-toothed Squirrel," from the "seven to ten minute parallel vertical grooves running down the front face of its incisors."[335]

The genus _Spermophilus_ includes a large number (forty or so) of Palaearctic and Nearctic animals known as Sousliks. The ears are small; there are cheek pouches as in _Tamias_. The general aspect of the animal is like that of a Marmot, and they bridge over the exceedingly narrow gap which separates the Marmots from the true Squirrels. Anatomically the skull is like that of {465} _Arctomys_; the molars are five in the upper and four in the lower jaw. The caec.u.m is relatively speaking very small; the measurements in a specimen of _S. tredecimlineatus_, dissected by Dr.

Tullberg, were: small intestine, 580 mm.; large intestine, 170 mm.; and caec.u.m, 27 mm. In _Tamias_ also the caec.u.m is not greatly developed. These animals are burrowing in habit.

The Prairie-dogs, genus _Cynomys_, of which the best-known species is perhaps _C. ludovicia.n.u.s_, are very like the Squirrels, but they are not arboreal creatures; they live in burrows on the ground, as their vernacular name denotes. The genus is entirely North American, and four species have been differentiated.