RODENTIA. The cranial cavity is depressed, elongated, and rather small, and the cerebral fossa lies entirely in front of the cerebellar fossa. The occipital plane is vertical or directed somewhat backwards, and the supra-occipital does not form much of the roof of the cranium. The paroccipital processes of the exoccipitals are generally of moderate size; in the Capybara (_Hydrochaerus_), however, they are very long, and are laterally compressed and directed forwards. The parietals are small, and often become completely fused together; there is sometimes a small interparietal. The frontals in most genera have no trace of a postorbital process; in Squirrels, Marmots and Hares, however, one occurs, but in no case does it meet a corresponding process from the zygomatic arch, so the orbit and temporal fossa are completely confluent. In Hares the postorbital process of the frontal is much flattened, and has an irregular margin. The temporal fossa is always small, and in _Lophiomys_ is arched over by plates arising respectively from the parietal and jugal; a secondary roof is thus partially developed in a manner unique among mammals, but carried to a great extent in many Chelonia. The nasal bones and cavities are large, attaining their maximum development in the Porcupines (fig. 98, 1). The premaxillae is always very large, and sends back a long process which meets the frontal. The vomer is occasionally found persisting in two separate halves, a feature recalling the arrangement in Sauropsids. In many Rodents there is an enormous vacuity at the base of the maxillary portion of the zygomatic arch. It is sometimes as large as the orbit, and attains its maximum development in the Capybara and other Hystricomorpha; in the Marmots, Beavers, and Squirrels (Sciuromorpha), and in the Hares it is undeveloped. In _Lagostomus_ the maxillae bears an upwardly directed plate of bone, shutting off from this vacuity a space which is the true infra-orbital foramen. [Illustration FIG. 98. HALF FRONT VIEW OF THE SKULL OF A PORCUPINE (_Hystrix cristata_) × 1/2. (Camb. Mus.) 1. nasal. 2. maxillo-turbinals. 3. infra-orbital vacuity. 4. maxillae. 5. premaxillae. 6. jugal. _i_ 1. upper incisor.]