136 Wortman — Studies of Eocene Mammalia in the 



of more than specific importance, and these constitute in my 

 estimation the main characters upon which the generic dis- 

 tinction rests. 



In the molars the anterior cusp of the trigon is distinct in 

 all, but least so in the last. There is likewise a very faint 

 indication of a posterior median cusp in the heel of the first 

 and second molars in the type, bnt in other specimens it is 

 apparently absent. The posterior portion of the crown does 

 not widen so rapidly as is the case in Omomys Carteri, the 



129 



11 12 



130 



Figure 129. — Left lower jaw of Hemiacodon gracilis Marsh; external 

 view ; two and one-half times natural size ; drawn from two specimens. 



Figure 130. — Crown view of the same specimen. 



In the drawing the alveolus of the first incisor does not appear as large as it 

 actually is. The upper portion of the alveolus is broken away, so that only 

 the bottom of the cavity is shown. 



transverse diameter of the anterior and posterior moieties being 

 more nearly equal. The fourth premolar is moi;e advanced in 

 structure than the corresponding tooth of 0. Carteri. The 

 third premolar in the type shows no trace of an internal cusp, 

 but in some other specimens in the collection, of which forty or 

 fifty individuals are represented, there is a distinct rudiment of 

 this structure to be seen. 



The second premolar and canine, figures 129 and 130, are 

 preserved in several specimens, and their reduced size, as com- 

 pared with the third premolar and the corresponding teeth of 

 0. Carteri, is very evident. The incisors are not preserved in 

 any specimen in the collection, but in several the front part of 

 the jaw is sufficiently complete and well preserved to permit the 

 alveoli to be made out with certainty. From these the number 

 is shown to be two, of which the first is considerably larger 

 than the second. The teeth were implanted in an uninterrupted 



