Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/439

Rh KODENTIA.] MAMMALIA 417 The tongue presents little of that variability in length observable in the preceding orders ; it is characteristically short and compressed, with an obtuse apex never protruded beyond the incisors ; in most species there are, as in Insectivora, three papillae circumvallatse at the base ; and the apical portion is generally covered with small filiform papillae, some of which in the Porcupines (Hystrix) become greatly enlarged, forming toothed spines. The stomach varies in form from the simple oval sac of the Squirrel to the complex ruminant-like organ of the Lemming. In the Water- Vole (Arvicola amphibius) and in the Agouti (Dasy- procta agouti) it is strongly constricted between the oesophagus and pylorus ; in the common Dormouse the oesophagus immediately before entering the stomach is much dilated, forming a large egg-shaped sac with thickened glandular walls, and in some other species, as in Lophiomys imhausi and in the Beaver, glandular masses are attached to and open into the cardiac or pyloric pouches. All Rodents, with the exception only of the species of Dormice (Myotxidx), have a caecum, often of great length and saccul- ated, as in the Hares, Water- Voles, and Porcupines, and the long colon is in some, as in the Hamster and Water- Vole, spirally twisted upon itself near its commencement. The liver is typically divided in all, but the lobes are variously subdivided in the different species (in Capromys they are divided into minute lobules); and the gall-bladder, though present in most, is absent in a few. In most species, as in many Insectivora, the penis (which is generally provided with a bone) may be more or less completely retracted within the fold of integument surrounding the anus, and lie curved backwards upon itself under cover of the integument, or it may be carried forward some distance in front of the anal orifice, from which in the breeding season, as in the Voles and Marmots, the prominent testi- cular mass separates it. The testes in the rut form projections in the groins, but (except in Lagomorpha) do not completely leave the cavity of the abdomen. Prostatic glands and, except in Lagomorpha, vesiculse seminales are present in all. The uterus may be double, each division opening by a separate os uteri into a common vagina, as in Leporidse, Sciuridee, and Hydrochcerus, or two-horned, as in most species. The mammary teats vary in number from the single, abdominal pair of the Guinea-Pig to the six thoracico-abdominal pairs in the Rats. In the Octodontidee the teats are placed high up on the sides of the body. The peculiar odour evolved by many Rodents is due, as in the Insectivora, to the secretions of special glands, which may open into the prepuce, as in Mus, Arvicola, Cricetus, &amp;lt;fcc., or into the rectum, as in Arctomys and Aulacodus, or into the passage common to both, as in the Beaver, or into pouches opening near the anus, as in the Hare, Agouti, and Jerboa. The integument is generally thin, and the panniculus carnosus rarely much developed. The fur varies exceed ingly in character, in some very fine and soft, as in the Chinchillas and Hares, in others more or less replaced by spines on the upper surface, as in the Spiny Rats and Porcupines ; in several genera, as in Xerus, Acomys, Platacantkomys, Echiothrix, Loncheres, and Echinomys, the spines are flattened. In the muscular structures the chief peculiarities are noticeable in the comparatively small size of the temporal muscles, generally so largely developed in Insectivora, and in the great double masseters, which are the principal agents in gnawing ; the digastrics also are remarkable for their well-defined central tendon, and in many species their anterior bellies are united between the mandibular rami ; the cleidomastoid generally arises from the basi-occipital, and the pectoralis major is connected with the latissimus dorsi ; in the Porcupines and Hares the tendons of the flexor di;itorum longus and flexor hallucis longus are connected in the foot, while in the Rats and Squirrels they are separate, and the flexor digitorum longus is generally inserted into the halluceal metatarsal. (See Dobson, Journ. Anat. Phys., vol. xvii.) SUBORDER I. RODENTIA SIMPLICJDENTATA. Rodents with two incisors only in the upper jaw, having their enamel confined to their front surfaces. The incisive foramina are moderate and distinct ; the fibula does not articulate with the os calcis; and the testes are abdominal, and descend periodically only into the inguinal canal. Section I. SCITJROMORPHA. Zygomatic arch slender, chiefly formed by the malar, which is not supported by a long maxillary process extending backwards beneath it; post-orbital processes present or absent; infra-orbital FIG. 92. Skull of Arctomys monax. opening small (except in Anomalurus) ; mandible with the angular part arising from the inferior surface of the bony socket of the lower incisor ; clavicles well-developed ; fibula distinct. Family 1. ANOMALURID^;. Arboreal Rodents, having their limbs connected by a cutaneous expansion supported by a cartilaginous process arising from the olecranon ; with a long hairy tail having large scales on its inferior surface near its root ; with sixteen pairs of ribs, and without post- orbital processes of the frontals ; pm $ ; molars not tuberculate, with transverse enamel folds. Ethiopian. I fpg FiG. 93. Anomalurus fulgens (reduced). Alston, Proc. Zool. Soe., 1875. Anomalurus, with (?) five species from West Africa, alone re presents the family. The peculiar caudal scales, which evidently assist the animal in climbing, and the position of the cartilaginous support of the parachute, are well shown in the above woodci (fig. 93). XV. - 53