XIII 



PHYLUM CHORDATA 



517 



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 

 Hcromion. 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 metatarsals of the three digits are fused 

 together. 



Skeleton of the Insect! vora. The neural spine of the axis 

 is usually well developed, that of the remaining cervical vertebrae 

 small or obsolete. The number of trunk vertebrae 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 sternebrae. 



The skull (Fig. 1104) varies greatly in the different families, 

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



Km. 1104. Skull of Tenrec (Centetes ccaudatus). fr. frontal; max. maxilla; pa. ' parietal ; 

 p. max. pre-maxilla ; sq. squamosal. (After Dobson.) 



paratively large cerebral fossae, 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 fossae are completely 

 continuous ; the zygoma is incomplete, and the tympanic does not 

 usually form a bulla. 



