532 ZOOLOGY SECT. 



the different families ; in some it is very short, but it is elongated 

 in many (the Porcupines, Squirrels, and Beavers). The sternum 

 of the Rodents has a long and narrow body ; sometimes there is 

 a broad presternum ; the posterior end is always expanded into 

 a cartilaginous xiphisternum. 



The skull is elongated, narrow in front, broader and depressed 

 behind. The nasal cavities are very large, especially in the Por- 

 cupines, with air sinuses in the upper part. In some the optic 

 foramina fuse into one. An interparietal is often present. Par- 

 occipital processes are developed. The orbit and the temporal 

 fossa are always continuous. The nasal bones are large, and the 

 nasal apertures are terminal or nearly so. The premaxillae are 

 always very large. A remarkable feature of the skull is the presence 

 in many of a large opening corresponding to the infra-orbital foramen. 

 The middle part of the zygoma is formed by the jugal ; the latter 

 often helps to bound the glenoid cavity, as in the Marsupials. The 

 palate is short, and the anterior palatine foramina large. The 

 periotic and tympanic may become ankylosed together, but not to 

 the neighbouring bones. The coronoid process of the mandible is 

 sometimes rudimentary or absent ; the angle is often produced 

 into a process. 



The scapula of the Rodentia is generally long and narrow. The 

 spine sometimes has a metacromion process and a long acromion. 

 The coracoid process is small. The clavicle varies as regards its 

 development. Vestiges of the sternal end of the coracoid are 

 sometimes distinguishable. There is considerable variation in the 

 bones of the arm and fore-arm. The radius and ulna are in most 

 instances distinct, though in close and firm apposition. The 

 scaphoid and lunar are usually united ; the centrale is sometimes 

 present, sometimes absent. The pelvis and femur vary greatly. 

 Sometimes there is a third trochanter. The fibula is sometimes 

 distinct, sometimes fused with the tibia. In the Jerboa the meta- 

 tarsals of the three digits are fused together. 



Skeleton of the Insectivora. The neural spine of the axis 

 is usually well developed, that of the remaining cervical vertebrae 

 small or obsolete. The number of trunk-vertebras varies in the 

 different families from eighteen to twenty-four, and there is also 

 great variation in the development of the various processes. The 

 caudal region varies in its length ; frequently it has chevron 

 bones. The presternum is expanded, the mesosternum composed 

 of distinct, narrow sternebrse. 



The skull (Fig. 1185) varies greatly in the different families, 

 in the higher forms approaching that of the Lemurs, with com- 

 paratively large cerebral fossse, large orbits with complete or 

 nearly complete bony rims, well-developed zygoma, and a tympanic 

 bulla and tubular auditory meatus. In the others the cranial 

 capacity is less, and the orbits and temporal fossse are completely 



