8 DESCRIPTION OP A SKULL OF 



borders of the foramina rotunda [the locations of these foramina are indicated by 

 asterisks (**)], and thus at or near the narrowest portion of the cranium. 



The pterygoid processes converge more rapidly forward in M. jeffersoai {I.e., 

 PI. Ill) than in J\f. leidyi (Pi. Ill), their most approximated points in the foi'mer 

 being just below the said foramina, but in the latter, at least 5 cm. more forward. 

 Their roots bend more outwardly, their horizontal interior portions are narrower, and 

 the anterior end of the basi-sphenoid, exposed to view between their inner margins, 

 is broader in the line of the section in M. jeff'ersoni than in M. leidyi. The area be- 

 tween the root of the pterygoid process and the inferior border of the temporal fossa, 

 in M. jeffersoni, overhangs the said process, sloping upward and outward about 30°, 

 and extending as a plane in that direction about 3 em., when it suddenly bends ver- 

 tically upward, and finally makes a sudden turn inward and upward to its margin on 

 the sagittal crest. The same surface in M. leidyi rises at once from foramen rotiin- 

 dum more than 60°, and curves gently up to the crest without any sudden bend. 

 Sections 5 and 6 will show these differences plainly. 



Dentition. — The left canine molar, as much as is left of it, as well as the alveoles 

 of these molars, are but very slightly curved and are of uniform diameter. This 

 species thus belongs to Group "B," in Cope's Synoptic Table (E. D. Cope: Pre- 

 liminary lieimrt on ilie Vertehrata discovered in the Port Kennedy Bone Cave, Am. 

 Phil. Soc, Yol. xii, 1871, p. 85). The group comprises but two other known species, 

 viz., M. ivheatleyi Cope, and M. dissimilis Leidy. Cope makes the following dis- 

 tinction between the two : 



" Molars triangular ; canine molars less compressed — M. ivlieatley iy 

 " Last molar oval ; canine molars more compressed — M. dissimilis^ 

 The third species of the group should be characterized thus : 

 Last molar ovato-triangular ; the others quadrangular ; canine molars less com- 

 pi'essed — M. leidyi. 



It might be added that the exterior dentine layer, in both of the former species, 

 is thinnest at the bulge, whereas in M. leidyi it has its maximum thickness at the 

 bulge. 



The figures of the teeth (PI. V, Figs. 7-9) are drawn with particular care. It 

 should be remembered that the triturating surfiices are preserved only of the 1st and 

 2d molars in the left maxilla, and of the 2d, 3d (partly), and 4th in the right maxilla, 

 and that the two 5th molars are broken off' so high up as to expose their pulp cavities 

 (compare PI. III). Even regardless of this, the teeth are not perfectly symmetrical. 

 In naming this species for the venerable palaeontologist, Prof. Joseph Leidy, 

 M.D., LL.D., of the University of Pennsylvania, I make but a small acknowledg- 



