74 THE WASATCU AND DIUDGER FAUNiE. 



lateral processes. The dorsal fin originates in front of the anal. In the 

 typical forms, teeth are well developed in a single series on the dentary, 

 premaxillary, and maxillary bones ; but, in the small forms, they are 

 invisiljle. ilouth moderate. 



There are two sections of this genus, the species of which differ in the 

 fomi of the dorsal scuta. In section I, these shields are transverse, and 

 their posterior borders are pectinate, a median tooth being especially prom- 

 inent. In section II, the scuta are not wider than long, and have but one, 

 a median tooth, which is the extremity of a median longitudinal carina. 

 The species of section I are D. dentatus, D. analis, D. theta, and D. pectorosxis ; 

 those of section II are D. humilis and D. altus. The species of section I 

 display a longer anal fin than those of section II. 



The species of this genus were more numerous in individuals than all 

 others combined, during the period of the Green River Lake. 



DiPLOMYSTUS DENTATUS Cope. 



Bulletin U. S. Geol. Snrv. Terra., 1877, p. 809. 



Plate X, fig. 1. 



Fin-radii: D.I — 13; A. I. 35. Vertebi'ee : dorsal, 18; caudal, 21. 

 The greatest depth enters the length without the caudal fin two and a half 

 times, and the head enters the same nearly three and one-third times. The 

 eye is large, its horizontal diameter a little exceeding the length from its 

 border to the inferior edge of the premaxillary bone, and is a little greater 

 than one-fourth the length of the head. - The premaxillary and dentary 

 bones are short and deep, the latter with a deep notch on the anterior border; 

 both are directed upwards. The maxillary bone is long and narrow, and 

 curved backwards at its lower end, which reaches a point below the anterior 

 border of the orbit. The profile behind the premaxillary bone is nearly 

 horizontal ; above the posterior part of the orbit, it rises, and a compressed 

 supraoccipitivl crest carries it to the gently convex dorsal line. The abdo- 

 men is convex, and is about as long as the caudal region. The last dorsal 

 ray rises above a point anterior to the first anal ray. The caudal is deeply 

 forked. The ventrals originate at a point barely in advance of a vertical 

 line from the first dorsal ray. The pectoral fins are short. The scuta of 



