200 PROCEEDINGS OF THE ACADEMY OF [1898. 



the difference in age they are more than one-third smaller. In the 

 fossil specimens the permanent incisor exceeds the grinders both in 

 triturating area and in alveolar depth and calibre. In rosmarus 

 and obesus this tooth is much smaller than — . The posterior molar 

 ("id!) in the fossil skulls (excepting the one belonging to the New 

 Jersey Geological Survey), 7 is a deeply rooted tooth of nearly the 

 same calibre and triturating area as n ^ 2 . In both rosmarus and 

 obesus it is the smallest and shallowest rooted of the permanent den- 

 tition and is sometimes wanting in aged specimens. 



Another marked character which is peculiar to the two fossil 

 specimens belonging to the collection of the Academy, and in some 

 degree to the specimen of the Geological Survey, is the remarkable 

 median depression of the maxillae at the incisive foramina, and the 

 constriction of tlie inter-incisor space. In the Sable Island example 

 the depth of the incisive foramina below the crown of °^i is 64 mm. 

 and the distance between the alveoli of the permanent incisors is 

 only 17 mm. In the Long Branch skull it is about the same. In 

 the recent Greenland specimen these measurements are 41 mm. and 

 30 mm., and in the Alaskan specimen 50 mm. and 30 mm. 



While the foregoing comparisons were made with an amount of 

 material far more comprehensive than that accessible to Dr. Leidy, 

 and supplemented by the valuable diagnoses, figures and tables of 

 measurements given by Dr. J. A. Allen, the author regrets that a 

 larger series of adult crania of our living species of walrus was 

 not available. Premising, however, that the recent skulls used in 

 this comparison are typical in essential characters of rosmarus and 

 obesus, and that the fossil skulls, including the type of virginianus, 

 all represent a period of Pleistocene Age, the writer concludes that 

 the weight of evidence favors the separation of the fossil and recent 

 species of Atlantic walrus under the following diagnoses: 



Rosmarus rosmarus (Linnasus). Recent Atlantic Walrus. 



Phoca rosmarus Linnreus ; Syst. Nat., 1758, p. 38. 

 Rosmarus rosmarus Rhoads; Airier. Nat., 1894, p. 523. 



Characters. — Ratio of greatest anterior maxillary width to the 

 paroccipital expansion, as 2 to 3 ; permanent upper incisor much 

 smaller than — ; — 3 smallest, shallow-rooted, in old adults some- 

 times absent; superior grinders relatively weak, the opposing rows 

 separated by a wide incisive diastema twice the width of the largest 



7 In this the posterior molars have fallen out, the alveoli showing them to 

 have been of large calibre but quite shallow. 



