DISTINCTIVE CHARACTERS. 567 



and heavy iii P. vitulina, to give room and support for the 

 thick strong teeth ; in P. fcetida it is short and narrow, and 

 uniformly tapering; in P. grcenlandica the muzzle is narrow, 

 rather lengthened and attenuated. The teeth of the molar 

 series in P. vitulina are relatively about two and a half to three 

 times larger than in either of the other species. 



In respect to other characters of the skull, the orbital fossae/ 

 are relatively larger in P. fcetida than in either of the others, 

 with the inner wall more deeply excavated, and the zygomatic 

 border rather angular (sometimes very markedly so) instead of 

 regularly convex. P. vitulina differs still further in the greater 

 development and inward curvature of the malar process of the 

 zygomatic arch. Another striking difference is seen in the gen- 

 eral contour of the upper surface of the skull, which in P. vitulina 

 is rather sharply convex, with (in old males) the ridges formed 

 for the attachment of the masseter muscles closely approxi- 

 mated along the median line, or sometimes actually meeting to 

 form a low, broad, incipient sagittal crest, while in both P. 

 grcenlandica and P. fcetida the whole top of the skull is nearly 

 flat, and the ridges for the attachment of the masseter muscles 

 form a thickened line at the edge of the skull where the lateral 

 and dorsal surfaces meet at a rather sharp angle. 



The lower jaw in P. vitulina is very heavy and short ; the 

 symphysis is very short, behind which the rami abruptly bow 

 outward and widely diverge ; the rami are very thick, with the 

 axis of expansion nearly vertical, and there is no inward cur- 

 vature of the inferior border. In P. grcenlandica the lower jaw 

 is very slender with a rather long symphysis ; the rami are very 

 thin and broad, the inferior borders of which curve inward so 

 as to nearly or quite meet for one-third of the length of the jaw, 

 or nearly as far back as the last molar, while the plane of verti- 

 cal expansion is very oblique. The lower jaw in P. fcetida quite 

 nearly resembles, in general form, that of P. grcenlandica. In 

 P. vitulina the vertical diameter of the ramus just behind the 

 last molar is only about two and a half times greater than the 

 transverse is at the same point, while in P. grcenlandica it is 

 fully four times greater. P. vitulina also differs from the others 

 by the abrupt angle formed by the ascending ramus.* 



* As will be noticed later (infra, p. 573) the lower jaw in P. vitulina varies 

 greatly in form and stoutness with age, and probably also with sex. In the 

 foregoing comparison the lower jaw of a very old male P. vitulina has been 

 compared with others corresponding in age and sex of P. grcenlandica and 

 P. fcetida. 



