482 University of California Publications in Geology [Vol.7 



are much shortened anteroposteriorly. Actual numerical pro- 

 portions in the sea and river forms follow : 



Latax Lutra 

 lutris nereis canadensis 



Length of trunk measured around curve, from 

 posterior side neural arch of 7th cervical to 

 anterior edge of sacrum, near the median 



line 412 mm. 390 



Length of neck measured around curve, from 

 anterior side neural arch of atlas to posterior 

 edge neural arch 7th cervical vertebra (near 



median line) 69 110 



Katio length of neck to length of trunk 16.74 28.20 



This very materially reduced ratio is even less than that of 

 the seal (Phoca vituUna) which, in a specimen at hand, is nearer 

 that of the river otter. There is in the sea-otter no evident 

 elongation of the trunk as is sometimes noted in aquatic animals. 



Tail 



The tail shows a tendency toward reduction in the number 

 of elements, having in our specimen twenty-one vertebrae instead 

 of the twenty-four which are present in Lutra. Coues asserts 83 

 that Gerrard gives the number of caudal vertebrae as eighteen, 

 which indicates a still further reduction in the specimen or 

 specimens examined by him. The Stanford University example 

 studied by the writer has nineteen caudal vertebrae. The re- 

 duction in the number of tail elements would not be unexpected, 

 as the sea-otter uses its hind feet largely for propulsion. In 

 this respect progression in aquatic adaptation in the sea-otter 

 tends to parallel that of the seal, which possesses only twelve 

 caudal elements. Martin notes 34 the fact that in swimming the 

 hinder extremities are placed far back, exceeding the tail when 

 stretched out in the act of swimming. The tail is said to appear 

 placed between them almost as much as it is in the seals. 



The flatness of the tail of Latax when viewed from above 

 suggests that the organ is used as a means of elevation and de- 

 pression of the head while swimming, and as a directive organ. 

 Probably it assists also in diving. Snow (loc. cit.) says that 



33 Coues, E., op. cit., p. 331. 



si Martin, W. C. L., op. cit., p. 59. 



