488 University of California Publications in Geology [Vol. 7 



In aquatic animals increased flexibility of the vertebral 

 column is associated with a pelvis having a position more nearly 

 parallel to the vertebral column than in land forms. The pelvis 

 is also more extended posteriorly, and has a looser connection 

 with the sacrum and weaker pubic symphysis. The sea-otter 

 shows all these characters to a small extent. 



The characters of the pelvis which serve to differentiate it 

 from that of the river otter are thus clearly related to aquatic 

 adaptation. 



Fore Limbs 



The fore limbs in Latax are proportionately smaller than in 

 the fluviatile form of otter. The humerus is not so compressed 

 laterally. The space between the radius and ulna is slightly 

 greater than in the river form. The metacarpals and phalanges 

 are reduced. The tuberosities for muscle attachment, while not 

 prominent, would probably become more so as the animal grew 

 older, and so are probably not significant from the standpoint 

 of aquatic adaptation. The head of the humerus is pushed back 

 with reference to the shaft of the bone, so that the fore limb has 

 a slightly more posterior position than in the river otter. In the 

 seal the humerus, radius and ulna are much shortened, and the 

 head of the humerus is pushed still farther back, the articular 

 surface, instead of being at right angles approximately to 

 the shaft of the bone, being more nearly parallel to it, giving the 

 limb a still more posterior position. The olecranon of the ulna 

 is, in the sea-otter, slightly less prominent than in the river 

 animal, which modification is associated with the general reduc- 

 tion in the size of the limb. The metacarpals resemble the 

 phalanges. In Latax as in Lutra, the fifth digit of the forefoot 

 is longer than the first. 



In the seal the forefeet serve mainly as paddles, while in the 

 sea-otter the almost unanimous testimony of observers is that they 

 serve as organs of prehension as, for example, holding food or 

 the young. 



The principal organs used by aquatic groups for locomotion 

 through the water are ( 1 ) posterior paddles, as in the seals, or 

 (2) a sculling tail, as in whales. The sea-otter evidently makes 



