Thorpe — New Genus of Oligocene Hycenodontidce. 277 



Akt. XXVI. — A New Genus of Oligocene Hycenodontidce; 

 by Malcolm Ruthekfokd Thorpe. 



[Contributions from the Othniel Charles Marsh Publication Fund, Peabody 

 Museum, Yale University, New Haven, Conn.] 



Table of Contents. 

 Introduction. 

 Description of species. 



Neohy amotion, gen. nov. 



N. Iwrritius (Leidy). 



Hycenodon Laizer and Parieu. 



H. cruentus Leidy. 



H. montanus Douglass. 



H. crucians Leidy. 



H. leptocephalus Scott. 

 Synoptic table. 

 References. 



Introduction. 



All of the North American species of hysenodonts are of 

 Middle Oligocene age. This group embraces diverse 

 forms, one of which it is now proposed to place in a new 

 genus, herein designated as Neohycenodon. The Yale 

 specimen, Cat. No. 12766 (see figs. 1 and 2), and Leidy 's 

 "Hycenodon horridus," figured in 1869 (pi. Ill), are 

 selected as cotypes. 



Three skull forms are represented among the American 

 Hyauiodontidse : (1) the strongly dolichocephalic form, 

 which is typical of Hycenodon, found in species seldom 

 exceeding the wolf in size; (2) the mesaticephalic form, 

 typified in the New World by H. paucidens ; and (3) the 

 large dolichocephalic type with a great range of verti- 

 cal jaw movement and a considerable development of 

 the canines, with skull modifications, similar in many 

 respects to those of the sabre-tooth cat Smilodon. This 

 third group is the one now termed Neohycenodon by the 

 author. The first group might well be subdivided, sep- 

 arating out the species with the extreme posterior open- 

 ing of the choanse, the group that is represented by 

 H. leptocephalus. 



The smallest species of Hycenodon now known, 

 H. mustelinus, is apparently not represented in the Yale 

 Collection. The short-faced group, of which H. pauci- 

 dens is the only American example, is by no means plenti- 

 ful in this Museum, whereas there are many individuals 

 of the long and slender-jawed type, representing all but 



Am. Jour. Sci.— Fifth Series, Vol. Ill, No. 16.— April, 1922. 

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