PHYLUM CHORDATA 



540 



raetacromion 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 metatarsals 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-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 sicull (Fig. 1173) varies greatly in the different families, 

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

 paratively large cerebral fossae, large orbits with complete or 



Fig. 1173.— Skull of Tenrec (^Centetes ecaudatus). Jr. frontal ; max. maxilla ; pa. parietal ; 

 p. max. prcmaxilla ; sq. squamosal. (After Dobson.) 



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

 buUa 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. 



The pectoral arch also varies a good deal in the different 

 families of the Insectivora. In the true Moles and their allies 

 there is a remarkable bone of cuboid shape articulating ventrally 

 with the presternum and dorsally with the humerus, and only 



