548 CHORDATA. 
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 1s 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 4. (Ad nat.) 
Scapula. 
:Acromion. 
Coracoid. 
Distal Carpals. | | Uina 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, z.¢., 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 
penis and these small ischia of the porpoise perform a like 
