MUSEUM OF COMPARATIVE ZOOLOGY. 



25 



those of the eared seals, hut the}- are far less so than those of some of 

 the earless seals, especially Macrorhinus, in which they are more porous 

 than in some of the cetaceans. All the sternal segments in the walrus 

 are much less ossified than in the Otariadce ; in the former the first 

 and ninth are almost wholly cartilaginous, leaving but eight ossified. In 

 Eumelopias all are ossified, the first being also developed anteriorly 

 into a long bony point, and the ninth similarly developed posteriorly.* 



Hence the Olariadce differ from the walrus type not only in many de- 

 tails of structure, but radically in the general form and proportions of 

 the whole skeleton. 



Comparison with the Phoca vitulina. — The eared seals differ 

 vastly from the earless seals, as represented by Phoca vitulina, in almost 

 every feature. In addition to the well-marked differences of form ex- 

 isting between nearly all the principal bones, there are remarkable 

 regional variations which indicate a wide difference in the zoological 

 rank of the two types. In the eared seals the length of the cervical 

 and thoracic regions of the body, as compared with its whole length, is 

 much greater than in Phoca, but in respect to the lumbar and pelvic 

 regions the reverse of this obtains, these regions being most developed 

 in the Phocidoe.\ In the eared seals (Eumetopias and Callurhinus, which 

 represent the two leading types of the eared seals) the ratio of the length 

 of the cervical vertebra? to the whole length of the spinal column is as 19 

 to 100 ; in Phoca vitulina as 18 to 100. In the former, the ratio of the 

 length of the dorsal vertebra to the whole length of the spinal column 

 is as 44 to 100 ; in Phoca vitulina as 37 to 100. That of the lumbar to 



* See the detailed measurements of the skeletons of E. Slelleri and Cdlurhinus ursinus 

 given beyond. 



f The following table gives the dimensions (in mm.) and the proportions of the differ- 

 ent regions in E. Sttlleri, C. ursinus, P. vitulina, and the Alaska walrus. 



