476 THE VERTEBRATE SKELETON. 



bulla. There are no paroccipital or postglenoid processes. 

 The exoccipital is not perforated by the condylar foramen, a 

 very exceptional condition. 



The mandible has a high ascending portion, is rounded off 

 below and has no angle. The symphysial portion is long, 

 narrow, and spout-like, and the coronoid process is small. 

 The thyro-hyals are ankylosed with the basi-hyal, which is 

 connected with the large forked stylo-hyals by ligament only. 



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 LopJiiomys 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 premaxilla 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 



