498 DR. ST. G. MIVART ON TUE PINNIPEDIA. [May 19, 



truth of the hypothesis ; but, nevertheless, it may be well to 

 enumerate the anatomical reasons which might be advanced in 

 support of it : — 



(1) In the FhocidcB, as in Lutra, there is no alisphenoid canal, 



while in both Oturia and Ursus it is present. 



(2) In the Phocidce and Lutra the paroccipital and mastoid pro- 



cesses are not united by a prominent ridge of bone, while in 

 Otaria and Ursus they are so united. 



(3) In the Phocida and Lutra the mastoid process does not much 



depend ; in Otaria and Ursus it depends considerably. 



(4) The bulla of Ljutra could be easily made to resemble that of 



Phoca by giving a rounded form to the mastoid ; in both 

 genera there is the same sort of groove between the mastoid 

 and the tympanic. The bulla of Otaria, on the contrary, 

 is exceedingly like that of Ursus, and in both those genera 

 the sort of groove which exists between the mastoid and 

 tympanic in Lutra and Phoca, is absent. 



(5) The angle of the mandible is very large in Otaria and Ursus, 



while in Lutra and Phoca it is smaller. 



(6) The femur is very short in Lutra and Phoca ; it is con- 



siderably longer relatively in Otaria and Ursus. 



(7) In Lutra and Enhydra the floor of the orbit formed by the 



maxilla is very large, and it is also in Leptomjx, at least, 

 amongst the VhocidcB, while in others of that family it is of 

 moderate size. It is very small in Otaria and Trichechus, 

 as it also is in Ursus. 



(8) There are noteworthy defects of ossification in the cranial 



walls in Lutra and the PhocidcB. There are no such defects 

 in Ursus or Trichechus, while they are but of small extent in 

 Oturia. 



(9) The suborbital foramen is very large in Lutra and Phoca 



barhata and Trichechus. It is small in the Bears, and of 

 moderate size in most Otaries. 



On the other side it may be urged that : — 



(1) The postorbital process is formed entirely by the malar in 



Otaria, Lutra, and Ursus, while it is formed in part also by 

 the squamosal in the Phocida — as it may be in Canis. 



(2) There is a postorbital process to the frontal in Lutra and 



Ui-sus as well as in Otaria, while it is entirely, or all but 

 entirely, absent in the Phocidce. 



The Distribution of the Pinnipedia. 



The Pinnipeds are pretty equally divided between the Northern 

 and Southern Hemispheres, there being about 14 northern and 11 

 southern species. Of the 9 genera, 5 (namely Phoca, Halichwrus, 

 Monachus, Cystophora, and Trichechus) are northern forms, while 

 Stenorhynchus, Leptonyx, and Ommutophoca are all three exclu- 

 sively southern. Macrorhinus is equally divided, one of its species 

 being northern and the other southern ; while Otaria has three 



