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Show 498 DR. ST. G. MIVART ON THE 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 Phocida, as in Lutra, there is no alisphenoid canal, while in both Otaria and Ursus it is present. (2) In the Phocida and Lutra the paroccipital and mastoid processes 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 Liutra 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 aud 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 considerably 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 Leptonyx, at ieast, amongst the Phocida, 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 Phocida. There are no such defects in Ursus or Trichechus, while they are but of small extent in Otaria. (9) The suborbital foramen is very large in Lutra and Phoca barbata 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 Ursus as well as in Otaria, while it is entirely, or all but entirely, absent in the Phocida. 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, Halichcerus, Monachus, Cystophora, and Trichechus) are northern forms, while Stenorhynchus, Leptonyx, and Ommatophoca are all three exclusively southern. Macrorhinus is equally divided, one of its species being northern and the other southern ; while Otaria has three |