68 MEMOIRS OF THE CARNEGIE MUSEUM 
cingulum surrounding it in the former genus, but which is absent in Dinohyus. 
Furthermore, in the type of Ammodon the crown is somewhat higher and on the 
posterior face there are two mammilated ridges of considerable prominence 
Fic. 20. Type of Ammodon leidy- Fig. 21. External and Crown View 
anum Marsh. Internal and Crown View of M; of Neotype of Ammodon leidyanum. 
of Py. # nat. size. (Collection Yale % nat. size. (Collection Yale Museum, 
Museum, No. 12040.) No. 12040.) 
which take their origin, one at the postero-internal angle and the other at the 
antero-external angle of the heel, and continue to very near the apex of the crown 
(see fig. 20). In Dinohyus hollandi there is only a slightly uneven folding of the 
otherwise smooth enamel on the posterior face of the protoconid, and there are no 
decided ridges. My of Ammodon leidyanum is very 
similar to that in Dinohyus hollandi. In the for- 
mer the posterior heel (hypoconulid) is of rela- 
tively greater development (see fig. 21) than in the 
latter, and the tooth has a continuous cingulum on 
the antero-external and antero-internal faces, while 
the corresponding tooth in Dinohyus has the cingula 
represented on the postero-external, and but very 
slightly on the external and internal angles of the 
anterior face. The teeth of Ammodon leidyanwm are 
Fig. 22, External and Dorsal Viewot larger than thosein Dinohyus and represent an even 
Fragment of Astragalus of Ammodon? + more modified type than the latter. 
nat. size. (Carn. Mus. Cat. Vert. Foss., 
No. 1548.) Unfortunately the type of Ammodon is rather 
inadequate and consequently the basis of compari- 
son is very limited, but the characters enumerated above would seem to indicate that 
the geological horizon in which the New Jersey specimen was found, represents 
