416 MAMMALIA [RODENTIA. from the maxillaries, by the invariable presence of zygomatic arches, and by the wide unoccupied space exist ing between the alveoli of the incisors and the molar teeth, and (except in Lagomorplui) by the an tero posteriorly elongated glenoid cavity. Post-orbital processes of the frontals exist only in the Squirrel*, Marmots, and Hares ; in all other genera they are rudimentary or altogether absent ; the zygoma never sends upwards a corresponding process, and the orbit in all is freely continuous with the temporal fossa ; the lachrymal foramen is always within the orbital margin ; in many species the infra-orbital foramen is very large (in some as large as the orbit), and FIG. 90. Skull of Hystrix cristaia (juv.). t, temp rai muscle; m, massetcr, m , portion of masseter transmitted through the infra-orbital foramen, the superior maxillary nerve passing outwards between Hand the maxillary bone. transmits part of the masseter muscle ; the zygomatic arch is variously developed, and the position of the malar in it is used as a distinguishing character for grouping the families ; the nasals are, with few exceptions, large, and extend far forwards ; the parietals are moderate, and there is generally a distinct interparietal ; the palate is narrow from before backwards, this is especially pronounced in the Hares, where it is reduced to a mere bridge between the premolars, in others, as in the great Rodent Moles (Bathyerginse), it is extremely narrow transversely, its width being less than that of one of the molar teeth ; tympanic bullse osseae are always present and generally large ; in some genera, as in the Gerbilles (Gerbillinae) and Jerboas (Dipodinee), there are supplemental mastoid bullro which form great hemispherical bony swellings at the back of the skull (see fig. 89, Per] ; in these genera, and in the true Hares, the meatus auditorius is tubular and directed upwards and backwards. The mandible is characterized by its abruptly narrowed and rounded symphysial part supporting the pair of large incisors, as well as by the small size of the coronoid process and great development of the angular portion. The dental formula varies from i f, c , pin f, m (total 28) in the Hares and Rabbits to i , c , pm , m f (total 12) in the Australian .Water-Rats ; but in the vast majority of the species it presents striking uniformity, and may be set down typically as i -J-, c , pm ~ or , m |. In Lagomorpha only are there more than a single pair of incisors, and in them the additional pair are small and placed quite behind the middle pair, 1 and in this group alone does the enamel extend even partially to the back of the tooth; in all others it is restricted to its front surface, and so, by the faster wearing away of the softer structures behind, a chisel-shaped edge is always maintained. Both the upper and lower incisors are regularly curved, the upper slightly more so than the lower, and, their growth being continuous, should anything 1 Professor Huxluy remarks (Proc. Zool. Soc., 1880, p. 655) : " The deciduous molars and the posterior deciduous upper incisors of the Rabbit have long been known. But I have recently found that unborn Rabbits possess, in addition, two anterior upper and two lower deciduous incisors. Both are simple conical teeth, the sacs of which are merely embedded in the gum. The upper is not more than one- hundredth of an inch long, the lower rather larger. " prevent the normal attrition by which their length is regulated, as by the loss of one of them, or by displace ment owing to a broken jaw or other cause, the unopposed unused incisor may gradually curve upon itself until a complete circle or more has been formed, the tooth, perhaps, passing during its growth through some part of the animal s head. The molar teeth may be rooted or FIG. 91. Vertical and Longitudinal Section through Skull of Castor Jtber, showing the cerebral cavity, the greatly-developed tuibinal lamellae, the mode of implantation of the large ever-growing chisel-edged incisor, and the curved rootless molars. rootless, tuberculated or laminated ; and this diversity of structure may be noticed even in the same family. When there are more than three back teeth those which precede the last three have succeeded milk teeth, and must there fore be considered premolars. In some species, as in the Agoutis (Dasyproctidee), the milk teeth are long retained, while in the allied Cavies (Caviidx) they are shed before birth. There are generally nineteen dorso-lumbar vertebra (13 thoracic and 6 lumbar), and their forms vary in the different genera ; in the cursorial and leaping species the lumbar transverse processes are generally very long, and in the Hares there are large compressed hypapophyses. The caudal vertebrae exhibit as much variety as in Insectivora, from their rudimentary condition in the Guinea-Pig to their great size in the Jumping Hares and prehensile - tailed Porcupines. The scapula is usually narrow, with a long acromion ; the clavicles may be altogether absent or imperfect, as in the Porcupines, Cavies, and Hares, but in most species they are well developed; the humerus has no supra condylar foramen, and the fore arm bones are distinct; in most species the manus has five digits with phalanges normally developed, the pollex rarely rudimentary or absent. In contrast to the normal condition of the pelvis in Insectivora, we find here largely developed ischiatic and pubic bones, with a long usually osseous symphysis ; the femur varies considerably in form, and there is generally a well-defined third trochanter ; in the Sciurine and Hystricine Rodents the tibia and fibula are distinct, but in the Rats and other Murines, and iu the Hares, these bones are united, often high up ; the pes is much more variable than the manus, the digits varying in number from five, as in the Squirrels and Rats, to four, as in the Hares, or even three, as in the Capybarn, Viscacha, and Agouti ; in the Dipodidse the metatarsals are greatly elongated, and in some of the species, as in the Jerboas, they are ankylosed together. The mouth is divided into two cavities communicating by a constricted orifice, an anterior containing the large incisors and a posterior in which the molars are placed, the hairy integument of the face being continued inwards behind the incisors. This evidently prevents substances not intended for food getting into the mouth, as when the animal is engaged in gnawing through an obstacle. In the Hares and Pacas the inside of the cheeks is hairy, and in some species, as in the Pouched Rats and Hamsters, there are large internal cheek pouches lined with the hairy integn- mert, which open near the angles of the mouth and extend backwards behind the ears ; in the New- World Pouched
Rats (Gecmyidse) the pouches open externally on the cheeks.