ON SOME NEW AND LITTLE KNOWN CREODONTS. 
175 
If this specimen actually belongs to Protopsalis, there can be no doubt of the 
correctness of placing this genus among the OxyoBnidoe ■, its relations to Pterodan 
will be considered in another place. 
Genus HY^NODON. 
Four well marked species of Hycsnodon occur in the White River beds of North 
America ; of these H. horridus and H. crucians Leidy are accurately characterized. 
H. cruentus, however, has hitherto been imperfectly known and in his last publication 
Dr. Leidy inclines to the view that it is only a smaller variety of H. horridus} A 
very fine skull in the Princeton museum shows that the two are, nevertheless, distinct. 
Aside from the diflerence of size the face is more depressed than in H. horridus, the 
last lower molar lacks the external buttress on the anterior lobe, and the posterior 
lobe of the last upper molar is externally concave and strongly curved outwards, as 
in n. crucians. The fourth species, for which I propose the name ff. leptocephalus, 
is new. It is founded upon two excellent skulls belonging to the Museum of Com- 
parative Zoology, and for an opportunity to study these very interesting specimens I 
am indebted to the kindness of Professor Agassiz. 
This species is somewhat larger than H. crucians, which it resembles in dental 
characters and in having the cranial constriction in advance of the fronto-parietal 
suture. On the other hand the cranium is narrower and less rounded, and the posterior 
nasal canal much more prolonged, being enclosed by the whole length of the palatines 
and the pterj^goid plates of the alisphenoids. The American species may be tabulated 
as follows : 
I. Posterior nares opening between posterior part of palatines ; pterygoid plates of 
alisphenoids not in contact below. 
A. Cranial constriction in advance of fronto-parietal suture, H. cruciaris 
B. Cranial constriction at fronto-parietal suture. 
a. Face very deep; an external buttress on anterior lobe of 
last lower molar, H. horridus 
b. Face shallower; buttress absent, H. cruentus 
II. Palatines in contact throughout; pterygoid plates of alisphenoids 
meeting below, H. leptocephalus 
The structure of the skull of Hyoenodon has not been completely described as 
yet, and an attempt to do so may be of some value, even though repeating some 
points already determined by Dr. Leidy. In what follows H. cruentus is taken as a 
standard, reference being made to other species only when they depart from it in some 
particular. 
The basi-occipital is short, broad, very thin and slightly convex from side to side. 
The limits of the other occipital bones cannot be very clearly made out, but as a 
whole the occiput is low and broad, somewhat like that of the opossum; the con- 
^ Ext. Mam. Faun. Dak. and Neb., p. 48. 
