williston: restoration of platygonus. 25 



The species are certainly closely allied, as a careful study of 

 Leidy's late paper assures me, yet there are certain differences which 

 seem to be remarkable if they are only individual. In any event the 

 abundant material at my disposal, more than all previously known in 

 the genus, and all certainly belonging to one species, will merit a 

 minute comparison and description. The best material so far de- 

 scribed as belonging to the species is the young skull from Kentucky 

 described at length by Leidy, and the two adult female skulls de- 

 scribed and figured by the same author in his latest paper. 



In the present collection there are two young skulls, of nearly or 

 quite the same age as that described by Leidy as Euchoerus macrops, 

 and they show the following differences: 



The alveolar process for the inferior incisors is more projecting, 

 the diastema between the outer incisor and the canine being eleven 

 millimeters in length, while in the figure it is represented as almost 

 nothing, the incisor coming close up to the canine. The chin 

 is more convex and protuberant in our specimens. The post- 

 canine diastema is shorter, and the jaw much more robust in this 

 region, the lower margin is less straight, and the coronoid process 

 not as high. In the cranium the face is broader, the front less con- 

 vex the external meatus is situated less far back. In the present 

 species, the sagittal border is parallel or slightly ascending from the 

 plane of the molar contact, while in macrops the crest is in a plane 

 markedly descending. On the under side the difference is equally 

 apparent in the more slender facial portion of Leidy's specimen. 

 From the top view the same marked slenderness is seen. In our 

 skulls of the same age the face is more suddenly contracted just back 

 of the most anterior part of the molar suture. There is no cul de 

 sac, such as Leidy describes and figures, below the anterior margin 

 of the posterior nares, but, in its place, there is a concavity, above 

 the more convex portion which takes the place of the sharp crescentic 

 ridge. In the lower molars there is no indication of a basal ridge 

 between the transverse ridges, and the anterior and posterior basal 

 ridges are narrower. While there is a distinct heel-cusp to the second 

 lower molar in our skulls, the sharp ridge from the outer cusp behind 

 runs straight to the apex of the heel prominence and does not have 

 an incision. In the third molar, the last cusp is smaller, about as 

 large as it is in the preceding molar of macrops, and the ridge con- 

 necting it from the preceding outer cusp is incised about as it is in 

 that tooth, and the outer side is rounded, not angular. 



The differences from the adult skull, as will be seen in the figures, 

 consists in the greater flatness and straightness of the frontal and 

 parietal region, the more projecting incisor alveoli, the more vertical 



