The Cambridge Natural History Part 40

One of the most remarkable genera of this family is the little _Heterocephalus_ from Abyssinia and Somaliland. As Mr. Thomas justly remarks,[349] it "is a peculiar-looking little creature, about the size of the Common Mouse, but looking almost more like a tiny hairless puppy on account of its nearly naked skin, small eyes, and peculiar physiognomy."

Though apparently naked, there are numerous scattered hairs over the entire body, and the toes are fringed with stiffish hairs, which must be advantageous to a burrowing animal. There are two species, _H. glaber_ (originally described by Ruppell), and _H. phillipsii_, of which our knowledge is due to Mr. Thomas. The length of the entire creature including the tail is not more than 134 mm., both species being approximately of the same dimensions. Mr. Lort Phillips, the discoverer of the species which bears his name, writes "that this little creature, called 'Farumfer' by the Somali, throws up in places groups of miniature craters, which exactly resemble volcanoes in active eruption. When the little beasts were at {482} work, I used frequently to watch them, and found that the loose earth from their excavations was brought to the bottom of the crater, and sent with great force into the air in a succession of rapid jerks, and that they themselves never ventured forth from the shelter of the burrows."[350]

FAM. 4. SPALACIDAE.--"The Spalacidae," observes Dr. Blanford, "are sometimes called rodent moles, and resemble a mole in general aspect, having cylindrical bodies, short limbs, small eyes and ears, large claws, and a short or rudimentary tail." The existence of a spiral valve in the caec.u.m may perhaps characterise this family; but it has at present only been found in the two genera _Spalax_ and _Rhizomys_.

_Spalax_ has inconspicuous eyes and external ears. The tail is totally absent. The lower incisors are more developed than in other Rodents; they project in a bony sheath beyond the posterior end of the ramus of the lower jaw. The scapula is long and narrow. The large intestine is half the length of the small intestine. The animal seems to have only two pairs of teats, one pectoral the other inguinal.

_Spalax typhlus_ of Egypt, which is probably not different from the European form, makes extensive burrows, some of the branches being even 30 to 40 yards in length. In a "domical chamber," situated along the course of one of these burrows, Dr. Anderson found no less than 68 bulbs stored up.

Its eyes are mere black specks among the muscles, but they appear, however, to have a proper organisation. There are altogether eight species of the genus, which is entirely Palaearctic in its range.

The genus _Rhizomys_, including a number of species known as Bamboo Rats, is purely Oriental in range. _Rh. sumatrensis_ reaches a length of 19 inches; the better-known species, _Rh. badius_, is at most only 9 inches in length--in both cases the measurements are exclusive of the tail, which is a quarter to one-third of the length of the body, and is not scaly but nearly naked, with a few scattered hairs. The molars are three, and the incisors usually orange in colour; but sometimes the upper incisors are white as in _Rh. badius_. There are thirteen dorsal vertebrae. In _Rh.

pruinosus_ the large intestine is considerably longer than the small intestine; the lengths of the two sections of the gut are 42 and 30 inches respectively. In another {483} species the large intestine is slightly shorter than the small intestine. In _Rh. badius_ the two parts of the gut are almost exactly equal in length. There are three pairs of inguinal and two pairs of pectoral teats. The name _Rhizomys_ appears to have been given to the animals of this genus for the reason that they feed largely on roots. They burrow, and, like many other burrowing animals, feed in the evening. As is the case with other forms, _Rhizomys_ is said to burrow with the a.s.sistance of its teeth as well as of its claws.

[Ill.u.s.tration]

FIG. 237.--Bamboo Rat. _Rhizomys badius._ .

_Tachyoryctes_ is an African genus closely allied to the last. There are three Ethiopian species. It is mainly to be distinguished by the different pattern upon the grinding surface of the molars.

FAM. 5. GEOMYIDAE.--This family of burrowing Rodents is limited to North and Central America. The animals have cheek pouches, and small eyes and ears, in accordance with their mode of life. The claws of the fore-limbs are very strongly developed.

The genus _Geomys_ contains some eight species, which are Central and North American, not extending, however, far north. The incisors of the upper jaw are grooved with two grooves. There are three pairs of teats--one axillary, and the two remaining inguinal.

_Th.o.m.omys_, without grooves on the incisors, reaches to Canada in the north, and does not extend as far south as the last genus. {484}

Allied to this family, and indeed united with it by Tullberg, but kept separate by Thomas, is the

FAM. 6. HETEROMYIDAE.--The members of this family are also American, but are not confined to the northern-central regions of that continent, for the genus _Heteromys_ extends into South America.

The genus _Dipodomys_, with twelve species, is of a Jerboa-like form, as the following measurements of an example of _D. merriami_ will show. The length of the head and body was 85 mm.; of the tail 127 mm.; the hind-foot is 32 mm. It has but four toes. The hind-limb is longer than the front-limbs.

In _Perodipus_ the same form is exhibited. There are, however, five toes, and the sole of the foot is hairy. The axis vertebra and the two following vertebrae are fused together.

_Perognathus_ is a third genus. It has the same general slender form, but the tail is not so long, being but little longer than the body. The hind-limbs, too, are shorter. The teats of this and of _Perodipus_ are as in _Geomys_. The two remaining genera of the family are _Heteromys_ and _Microdipodops_.

[Ill.u.s.tration]

FIG. 238.--Jerboa. _Dipus hirtipes._ 1/3. Eastern Europe.

FAM. 7. DIPODIDAE.--This family consists of small, plain-living, and leaping or arboreal creatures, commonly known as Jerboas. The main anatomical characters of the family are the following:--There is a large infra-orbital foramen. The molars are always reduced, the premolar being either absent in the lower jaw alone or in both jaws. This family presents an obvious likeness to _Dipodomys_ (hence the name of the latter) and to some other members of the American family Heteromyidae {485} There is even the same ankylosis of the neck vertebrae. We find, moreover, the same a.s.sociation of long-legged and shorter-legged forms that characterises the Heteromyidae.

[Ill.u.s.tration]

FIG. 239.--Bones of right pes of Jerboa, _Dipus aegyptius_. . _a_, Astragalus; _c_, calcaneum; _c_^2, middle cuneiform; _c_^3, outer cuneiform; _cb_, cuboid; _n_, navicular; I-IV, first to fourth toes. (From Flower's _Osteology_.)

The typical genus _Dipus_ is a smallish quadruped with long naked ears and a long tail. The ten species are all Palaearctic in range. The fore-limbs are short and five fingered, and the short pollex has no claw; the hind-limbs are excessively long and only three-toed. The bony structure of these limbs is remarkable. The three metatarsals are elongated almost like those of a bird, and are ankylosed together. The digits have long phalanges which alone reach the ground as the animal hops. It is a curious fact, and one not so easily identifiable with the way of life, that the neck vertebrae of this genus are ankylosed together with the exception of the atlas, which is free; the arrangement is precisely like that of the Sperm Whale. The last vertebra is, however, sometimes free. The Jerboas not only leap but they burrow, and their strong incisors are said to be used in burrowing through stony ground. They are eaten by the Arabs, and are, or have been, called Daman Israel, _i.e._ Lamb of Israel. In _D. hirtipes_ the body and tail measure respectively 4 and 7 inches. The hind-feet have a tuft of long hairs below. Mr. W. L. Sclater's newly-founded genus _Euch.o.r.eutes_[351] is somewhat more primitive in its characters than is _Dipus_. The general form is the same, with long ears and a long tail. But there are five toes to the hind-limb, the two lateral ones though nailed being much shorter than the middle three. It has a "long pig-like snout,"

and the tail is cylindrical as in most other Jerboas, with a tuft of longer hairs at the end. The incisor teeth, grooved in _Dipus_, are here smooth, as in _Alactaga_. The species was probably obtained "in the sandy plains round the city of Yarkand." {486}

_Alactaga_ is much like _Euch.o.r.eutes_; it has five toes, a cylindrical tufted tail, the hairs at the end distichous, smooth incisors, and a premolar present in the upper jaw. It also differs from _Euch.o.r.eutes_ by the much smaller auditory bulla as well as in the fact that the infra-orbital foramen has no separate pa.s.sage for the nerve, which pa.s.sage is to be distinguished in both _Dipus_ and _Euch.o.r.eutes_. The best-known species is the Siberian Jumping Rabbit, _A. jaculus_. Beneath the ends of the three main toes of the feet are remarkable fan-shaped pads. In _A.

dec.u.mana_ the body and tail measure 7 and 10 inches respectively, the ears 2 inches. _Platycercomys_, a fourth genus of the family, is much less known and is to be differentiated from the last three genera by the fact that it has no premolars at all, the grinding tooth formula being thus 3/3. The tail too is flattened and "lancet shaped." It extends from Siberia to Nubia, and thus just enters the Ethiopian region.

The above are the more typical Jerboas. There remain several forms which are not at all Jerboa-like in their way of life, but are nevertheless, on anatomical grounds, placed with them. _Zapus_, an American genus, with the exception of one Palaearctic species, is transitional in that its hind-legs are rather long, but there is not so much difference between them as in the typical Dipodidae. _Sminthus_ is at the opposite extreme to _Dipus_. Its feet are short and of equal length; it climbs in trees, and may perhaps be looked upon as nearest of all Dipodidae to the ancestral form of the group.

FAM. 8. PEDETIDAE.--The genus _Pedetes_ contains but one species, _P.

caffer_, the Cape Jumping Hare. The animal suggests a large Jerboa in appearance on account of its jumping habits, the long hind-limbs, and the long tail. The length of a fair-sized example is some 17 inches, with a tail of the same length. The eyes and ears are large. The hands are five-fingered and the feet only four-toed, the hallux being of course the absent digit. In the skeleton it is interesting to note that the second and third cervical vertebrae are so close together that there can be no free movement; interesting because in _Dipus_ the cervicals are actually ankylosed. The dorsal vertebrae are twelve. The small intestine is long, measuring 7 feet 4 inches, while the caec.u.m is short, being only 8 inches long. The large intestine is 3 feet 10 inches long. The gall-bladder appears to be {487} absent,[352] an exceptional state of affairs in Rodents. A singular fact in the anatomy of this animal is the existence of a septum dividing the lower part of the trachea. This is sometimes met with in birds. As might be supposed from its large eyes, the Spring Haas, as the animal is sometimes called, is nocturnal. Its long hind-limbs permit it to leap enormous distances. It is a burrowing Rodent.

SECTION 3. HYSTRICOMORPHA.

FAM. 1. OCTODONTIDAE.--The Rodents of this family are of small to moderate size, the only, relatively speaking, giant in the family being the "Water-Rat," _Myocastor_. The toes are with but one exception not reduced; the tail is long in the majority of the genera. The teats are placed high up on the sides of the body. The clavicle is fully ossified. All the genera are South or Central American in range with the exception of _Petromys_.[353]

SUB-FAM. 1. OCTODONTINAE.--_Octodon_ has four species, which are all Chilian, Peruvian, and Bolivian in distribution. The Degu, _O. degus_, has a length of 160 mm., with a tail 105 mm. long. The ears are 18 mm. long. At the roots of the claws are longish and stiff hairs which appear to serve as "combs." The tail has long but spa.r.s.ely scattered hairs. There are twelve pairs of ribs. The lengths of the various sections of the intestine are as follows: small intestine, 680 mm.; caec.u.m, 90 mm.; large intestine, 390 mm.

These animals live in large companies. Closely allied is the genus _Habrocoma_ (more correctly, as it appears, to be written _Abrocoma_), with two species. _H. bennetti_ is 204 mm. long, with a tail of 103 mm. The ears are long, 22 mm. The fore-feet have no outward trace of the thumb. Stiff hairs like those that characterise _Octodon_ are found also in this genus.

The fur is very soft. The furring of the tail is much thicker than in _Octodon_.

_Spalacopus_ with but a single species, _S. poeppigi_, is a burrowing animal, from which indeed, and on account of its resemblance to _Spalax_, it has received its name. The ears in accordance with the underground life are short, only 5 mm. in length in an {488} example of 120 mm. The tail too is reduced, being in the same example only 42 mm. in length. As in the last two genera the large intestine is about one half of the length of the small intestine.

The "Tuco-tuco," genus _Ctenomys_, has also short ears and tail. The claws of the fore-feet are longer than those of the hind-feet.

A related form is _Aconaemys_ (better known as _Schizodon_), with similar external characters; it inhabits high localities on the Andes.

_Petromys_ is the only genus of the sub-family which is not American in habitat. It is an African form and there is but one species. Its anatomy conforms to that of the genera already considered. The main difference in structure is shown by the teeth. Their surface is uneven, and differs from that of other Hystricomorphs "in that the enamel to the inside of each upper jaw-tooth and outside on each lower jaw-tooth forms two tubercles, to which correspond grooves in the reverse position of the applied teeth."

SUB-FAM. 2. LONCHERINAE.--The genus _Echinomys_ with thirteen species belongs to the Neotropical region. The members of the genus are ent.i.tled "Spiny Rats" since they have spines mixed with the fur. The tail is long and the ears are very well developed. Both feet are five-toed. The tail is scaly as well as haired. _Trichomys_ (also called _Nelomys_) is very close to the above, and is also from the same part of the world.

The genus _Cannabateomys_ contains but one species, _C. amblyonyx_, which was formerly included in the genus _Dactylomys_, but has lately been separated by Dr. Jentink.[354] The animal is Brazilian and has a total length of 520 mm., of which 320 mm. belong to the tail. It is a climbing rat, and in accordance with that way of life has undergone some modifications. The fore-feet are four-toed, the two middle toes being markedly longer than the outer ones. The hind-feet are five-toed with the same greater development of the two middle toes. The claws are small and somewhat nail-like.

_Dactylomys_, also Brazilian, and with but one species, _D. dactylinus_, differs from the last in the fact that the molars are simpler in form; they are divided into two lobes, each of which {489} has but a single enamel fold, whereas in _Cannabateomys_ these teeth have several enamel folds. The tail, moreover, is but slightly hairy.

_Loncheres_ with eighteen species is another Neotropical genus allied to the foregoing. Small spines are, as in many of these genera, intermingled with the fur. This genus has as many as seventeen dorsal vertebrae, which is an unusually large number. _L. guianae_ is known as the "Porcupine Rat."

Allied genera, also South American, and without spines in their fur, are _Mesomys_, _Cercomys_, and _Carterodon_.

The South American _Thrinacodus_ is also known by one species,[355] _T.

albicauda_, which has rather more than the distal half of the long tail of a white colour. The fore-feet have four toes. The ears are broad and short.

SUB-FAM. 3. CAPROMYINAE.--A third sub-family of the Octodontidae is formed by the genera _Myocastor_, _Capromys_, _Plagiodontia_, and _Thrynomys_, which are all Neotropical forms with the exception of the last, which is African.

_Thrynomys_ (better known perhaps as _Aulacodus_) is a genus of African Rodent, containing some four species. The best-known of these is _T.

swindernia.n.u.s_, the Ground-Rat of West and South Africa. Its structure has been investigated by Garrod,[356] by Tullberg,[357] and by myself.[358] The fur is mingled with flattish bristles; the tail is moderately long, about half as long as the body. The fore-feet are five-toed, but the two toes at each end of the series are quite small. The hind-feet are only four-toed, the hallux being absent. The claws of the hind-feet are stronger than those of the fore-feet. The ears are not long. The limbs are decidedly short, hence the name of "Ground-Pig" sometimes applied to this animal. The molars are four in number in both jaws. The incisors of the upper jaw are twice grooved. There are thirteen dorsal vertebrae. The length of the small intestine is 60 inches, that of the large 49; the caec.u.m is short, being only 8 inches long. It is a remarkable fact that the acromion is joined to the rest of the spine of the scapula by a joint.

_Myocastor_, a name which seems to have the rights of priority over the more familiar _Myopotamus_, applies to a large South American aquatic Rodent. The general aspect of the animal {490} suggests a Water-Rat of large size (it has been exhibited in shows as a phenomenal product of London sewers!); the tail is nearly as long as the body. The ears are small. The limbs are short. The tail is naked. The hind-feet are webbed, but not so much so as in _Hydromys_. A small thumb is present. The animal has thirteen pairs of ribs; the molars are four in each jaw. The large intestine is more than three times the length of the small, and the caec.u.m is, as in the last genus, relatively short.

_Capromys_ is a genus[359] which is remarkable on account of its restricted distribution. It is found only in the islands of Cuba and Jamaica. There are four species, of which _C. melanurus_ is a dark brown-coloured animal with a blacker tail, nearly as large as a Rabbit. The native name of this Rodent is "_hutia_." It is also remarkable for having a stomach more complicated than is the rule among the mammals of this group. The organ is divided by two constrictions into three compartments. In _C. pilorides_ the liver is occasionally divided up in an extraordinary fashion into small lobules. _Capromys_ has the large number of sixteen dorsal vertebrae.

FAM. 2. CTENODACTYLIDAE.--For these African genera it seems admissible to form a distinct family, though Thomas, and Flower and Lydekker, only allow to the genera _Ctenodactylus_, _Pectinator_, and _Ma.s.soutiera_ sub-family rank. On the other hand, Tullberg removed these genera entirely from the Hystricomorph section and placed them as a section of the sub-tribe Myomorphi of the tribe Sciurognathi. It was chiefly the form of the mandible which led to this placing, for in these Rodents, as in all Squirrel- and Rat-like Rodents, and unlike what is found in the Hystriciform genera, the angular process of the mandible is not bent sideways.

The genus _Ctenodactylus_ derives its name from the peculiar strong bristles which form a comb-like structure upon the hind-feet and hide the claws; these are stated to be for the purpose of dressing the fur. The Gundi of North Africa, _C. gundi_, has a length of 190 mm., with a short tail of 17 mm. The ears are only moderate in size. The dental formula of the molars is 4/3. The incisors are white. The feet have four digits, and the hind-limbs are the longer. The large intestine is distinctly longer than the small intestine. {491}

_Pectinator spekii_ is the only representative of a genus not far removed from _Ctenodactylus_; it is a smallish Rodent, 6 inches in length, exclusive of a rather bushy tail nearly 3 inches long. It comes from Abyssinia. It has somewhat the appearance of a Squirrel, which is heightened by the fact that when sitting the tail is arched over the back; when running the tail is carried out straight. There are only four toes visible externally on both fore- and hind-limbs, but pollex and hallux exist in the skeleton, with a single phalanx each. There is only a single pair of mammae, and in correspondence with this but two or three young are produced at a time. The hind-feet have bristles very much like those of _Ctenodactylus_. The molars, however, are 4/4. There are twelve ribs, of which six reach the sternum. The latter is made up of six pieces, and the manubrium in its breadth anteriorly suggests that of the Vizcachas. The clavicles are present.[360]