48 ON THE EXTINCT MAMMALIA OF 



to H. cruentus may really belong to the former, as a small variety, or perhaps to 

 the female. 



The antero-posterior diameter of the isolated upper last molar is 11 lines; its 

 thickness anteriorly 5i lines; the length of its anterior cusp 8 lines; and the depth 

 of its back lobe posteriorly 3 lines. 



HTiENODON CRUCIANS. 



The remains of a third species of Hycenodon, from Dakota, distinguished by the 

 above name, indicate an animal smaller than the Hijcenodon leptorhynchus of France, 

 but rather larger than the recent Red Fox, Canis fulvus. 



The specimens of this species we have the opportunity of examining, derived from 

 four different individuals, are as follows : a much fractured and otherwise mutilated 

 skull, with the greater portion of the jaws and teeth ; a second skull less fractured, 

 but without the occipital portion, lower jaw and most of the teeth; fragments of a 

 third skull, consisting of the forehead and portions of the jaws with teeth ; and lastly 

 some small fragments of jaws with teeth from a fourth skull. 



Figure 1, plate II, represents a lateral view of the skull and lower jaw of the first 

 mentioned specimen, with a portion of the side of the face restored from the second 

 specimen indicated. 



Figures 2 and 3, of the same plate, represent superior and inferior views of the 

 second specimen. 



As observed in the upper view of the latter, the cranium of HycEnodon crucians 

 differs from that of R. liorrklus in being proportionately much less constricted and in 

 having the constriction much more advanced in position. Thus in H. liorridiis the 

 narrowest portion of the cranium is shortly in advance of the middle of the temporal 

 fossse; in H. crucians it is comparatively a short distance back of the forehead. In 

 consequence of this relative change in the position of the cranial constriction in the 

 two species, the posterior portion of the cranium in H. crucians appears proportion- 

 ately longer and more capacious than in H. Jiorridus, while the anterior portion 

 appears much shorter and of less capacity. 



The fronto-parietal suture, while descending at the sides of the narrowest portion 

 of the cranium in H. Jiorridus, is situated some distance back of the narrowest portion 

 in H. crucians. 



The forehead and face have the same form as in H. Jiorridus, but are better 

 preserved in the second of the specimens of S. crucians than in those of the former 

 species. The frontal bone, as seen in figure 2, is single, a condition probably due to 

 advanced age, the skull being an old one, as indicated by the remaining teeth, which 

 are much worn. In the other specimens of H. cruciaris and those of H. Jiorridus, 



