XIII 



PHYLUM CHORDATA 



513 



pr.sc 



In the Sloths (Fig. 1154) the arm-bones are comparatively long 

 and slender. A coraco-scapular foramen is formed as in the 

 Ant-eaters. In the three-toed Sloths (Fig. 1155) the acromion (acr.) 

 is at first connected with the coracoid process, but becomes reduced 

 and loses the connection ; in the two-toed Sloth the connection 

 persists. The clavicle (cl.) is not directly connected internally with 

 the sternum ; externally it is directly 

 connected with the coracoid process 

 a condition observed in no other 

 Mammal. The humerus is very long 

 and slender ; so are the radius and 

 ulna, which are capable of a certain 

 amount of movement in pronation 

 and supination. In the carpus (Fig. 

 1156) the trapezoid and magnum 

 are united in Bradypus, distinct in 

 Choloepus : in the former the tra- 

 pezium is usually fused with the 

 rudimentary first metacarpal. The 

 first and fifth metacarpals are re- 

 presented only by rudiments. The 

 proximal phalanges of the three 

 digits are early ankylosed with the 

 corresponding metacarpals, so that it 

 might readily be supposed that one 

 of the ordinary bones of each digit 

 was absent. 



The pelvis of the American Ant-eaters is elongated, with, a short 

 symphysis pubis. The ischia unite with the spinal column. There 

 is no third trochanter. The tibia and fibula are nearly straight, 

 and parallel with one another. In Cydoturus the pes is modified 

 to form a climbing organ. 



In the Sloths the pelvis is short and wide ; the spines of the ischia 

 unite with the anterior caudal vertebrae so that a sacro -sciatic 

 foramen is formed as in Ant-eaters. The femur is long and slender ; 

 it is devoid of third trochanter. The tibia and fibula are also long 

 and slender. At its distal end (Fig. 1157) the fibula develops a 

 peg-like process (x) which fits into a depression in the outer face of 

 the astragalus. The calcaneal process is extremely prolonged in 

 Bradypus, in which there is a tendency to ankylosis between the 

 tarsal bones, and the proximal phalanges ankylose with the meta- 

 tarsals. 



In the Armadillos the pelvis (Fig. 1158) is extremely long, and 

 both ilia and ischia are firmly fused with the spinal column. The 

 femur has a prominent third trochanter. The bones of the pes 

 (Fig. 1159) are normal. 



Skeleton of Cetacea. In the Cetacea (Fig. 1160) the cervical 



cor 



FIG. 1153. Shoulder-girdle of Arma- 

 dillo (Dasypus sexcinctus). acr. 

 acromion ; cor. coracoid process ; 

 pr.sc. pre-spinous fossa; pt. sc. post- 

 spinous fossa ; sp. spine ; sp'. ridge 

 probably marking the anterior 

 limit of origin of the subscapularis 

 muscle. 



