NO. 1080. THE FOSSIL GEXUS PTTLODVS—OIDLEY. gl7 



the outer roAv of cusps, as compared with 6 in P. montanus. The 

 upper dentition of this species can at present be compared only with 

 that of Ptilodus pUcatus Cope, since the upper molariform teeth of 

 other species of the genus are not known. The principal differences 

 thus shown are: (1) The smaller proportions of P. gracilis, which is 

 about one-fourth less in size; (2) the proportionately greater antero- 

 posterior length of the cheek teeth; and (8) the greater number of 

 cusps in p-", there being cusps instead of 5, as in the type, or 4, as in 

 the neotype of P. plicatus. The luimber of these cusps may be more or 

 less variable, but are probably of specific importance. 



DETAILED DESCRIPTIO'n. 



The dentition. — The teeth of the present specimen being fully adult 

 and but lit'tle Avorn afford an excellent opportunity both for a de- 

 tailed and general study of their characters. 



The single uj^per incisor preserved is placed in the premaxillary 

 near but not closely appressed to the median line. It is a relatively 

 long, rounded and gently curved tooth, with a pointed tip. The 

 crown is slightly compressed on the posterior face Avith lateral angles 

 dividing the surface into tAvo unequal areas, the posterior one being 

 the smaller and less convex. Near the summit of the tooth the pos- 

 terior area is subdivided obliquely by a sharp ridge running from the 

 aj:)ex upAvard and outAvard to the outer main angle, so that the tip 

 of the croAvn is roughly triangular in cross section. 



The next tooth in the upper series, which from its position is appar- 

 ently a canine, is separated from the incisor just described by a con- 

 siderable diastema. It much resembles the incisor in general form, 

 l)ut is smaller, and a posterior oblique ridge runs to the inner main 

 angle instead of the outer one. A considerable diastema separates 

 the canine from the premolar series. 



The first ujDper premolar, p^, is a triangular tooth composed of 

 three nearly equal subpj^ramidal cusps Avhose apexes point slightly 

 baclvAvard. Premolar- resembles p^, except that it is quadrate in 

 outline and has four subequal cusps. This tooth is somcAvhat higher 

 croAvned than the others and is the largest of the anterior three pre- 

 molars. Premolar"' equals p- in length, but is narroAver and less 

 robust. The crown is quadrangular and bears six cusps of nearly 

 uniform size arranged in tAvo longitudinal rows. The anterior pair 

 is someAvhat smaller than the others, and the cusps are closely joined 

 to those of the median pair. This tooth strikingly resembles the 

 upper true molars of Plagiaulax {Bolodon). All the cusps of the 

 anterior premolars have the peculiar wrinkled appearance noted by 

 Cope, Marsh, and others. 



In classing the next tooth of the upper series w^ith the premolars. 

 I differ from the opinions of Cope and Osborn, Avho called the cor- 



