548 CHORD ATA, 



unique. Clavicles are absent, but the scapula is large, flat 

 and broadened into a fan-shape. The prescapular fossa is 

 very small, the spine being bent forwards (aquatic type). 

 The humerus moves freely on the scapula, but this is the 

 only possible movement of the limb. The humerus is very 

 short and stout and bears two equally short flattened bones, 

 the radius and ulna. Six small carpal bones follow carry- 

 ing five digits. The digits are peculiar in having a greater 

 number of phalanges than is usual for mammals (2.3.3.3.3.). 

 This feature has formed a puzzle to morphologists ; a pos- 

 sible explanation of their multiplication is the formation of 

 supplementary phalanges from the epiphyses of the others 



Fig. 378. — Lateral View of Pectoral Girdle and Fin 

 OF A Porpoise x \, {Ad nat.) 



Scapula. 



Distal Carpals. | | Ulna. Humerus. 



Proximal Carpals. Radius. 



to meet the demand for increased surface. Some of the 

 phalanges present the anomalous feature of an epiphyses at 

 each end. The whole limb is firmly welded together by 

 fibrous tissue and little or no motion is possible at elbow or 

 wrist : indeed, in old specimens, the limb-bones are anky- 

 losed together. The shortening of the limb is due to the 

 same cause as in the mole, i.e., the need for a short, quick, 

 powerful stroke. 



The hind-limbs have entirely disappeared, leaving no 

 trace, and the pelvis is represented only by a pair of small 

 bones which represent the ischia. In terrestrial mammals 

 the ischia form a support for the cavernous bodies of the 

 j)enis and these small ischia of the porpoise perform a like 



