478 University of California Publications in Geology [Vol.7 



The general rounding of the profile of the sea-otter skull and 

 the increased width of the interorbital constriction result re- 

 spectively from the increased mass of the brain-case and the 

 enlargement of the skull as a whole. The lesser prominence of 

 the zygomatic process of the frontal and the reduction of the 

 angular process of the mandible may be related to aquatic adapta- 

 tion in that they signify a general reduction in the angular tuber- 

 osities for muscle attachment, which has been found by Raymond 

 Osburn 20 to be indicative of natatory modification. There might 

 theoretically be a reduction in certain angular tuberosities for 

 muscle attachment (those to which the supporting muscles are at- 

 tached) as an animal became adapted to life in water. The 

 medium does not require the development of such heavy muscles 

 as are required on land for support and locomotion, and so would 

 not necessitate the production of large processes for their attach- 

 ment. 



Dentition 



General Characters. — Upon comparing the teeth of Lutra and 

 Latax one is impressed immediately by the broader and more 

 rounded character of the cheek-teeth of the latter. Coues ex- 

 presses it well in saying : " If the teeth of ordinary carnivorous 



Figs. 5-6. Teeth of Latax and Lutra (natural size). 



Figs. 5a and 56. Milk carnassial, true lower earnassial and second 

 lower molar of Latax lutris nereis. Fig. 5a, superior view; 

 56, lateral view. 



Figs. 6a and 66. Lower carnassial and second lower molar of Lutra 

 canadensis ; Fig. 6a, superior view; 66, lateral view. 



26 Osburn, E. C, "Adaptive modifications of the limb skeleton in 

 aquatic reptiles and mammals," Ann. Acad. Sci. New York, vol. 16 

 (1906), p. 449. 



