730 THE EOCENE FAUNA. 



surfaces are not so rough as in the P. wMtei, as the granules are confined 



to the center of the scale, and the concentric lines are much more obvious 



and form a wider border. Ctenoid denticles distinct. Caudal fin openly 



emarginate. 



Meas^lrements. 



Depth of anterior edge of anal fin 0206 



Length from anterior edge of anal fin to end of caudal vertebrae 0305 



Length of caudal fin to vertebral centra 020 



Length of base of dorsal fin 0282 



Length of base of soft dorsal 015 



Length of base of anal - 017 



Length of base of spinous anal 008 



Length of ninth dorsal spine 013 



Length of fifth anal spine 015 



Depth of caudal peduncle (about) - Oil 



Remarks. — Among the known extinct genera of fishes it is Mioplosus 

 Cope that approaches nearest this one. The former is characteristic of the 

 Green River beds of the Lower Eocene. The genus Plioplarchus does not 

 enable me. to identify the horizon from which it is derived, with any of our 

 known formations. It only permits the general statement that its age may 

 be tertiary or upper cretaceous. 



REPTILIA. 



OPHIDIA. 



HELAGRAS Cope. 



Proceedings Amer. Philosoph. Society, 1883, p. 545. 



The generic characters are drawn from vertebrae only. These display 

 a modified form of the zygosphen articulation, as follows : The roof of the 

 zygantrum is deeply notched on each side of the median line, so as to ex- 

 pose the superior lateral angles of the zygosphen. This separate median 

 portion of the roof of the zygantrum forms a wedge-shaped body, which 

 may be called the episphen. It is surmounted by a tuberosity, which con- 

 stitutes the entii'e neural spine. The latter is thus entirely different in form 

 from that of other serpents. Articular extremities of centrum round, the 

 ball looking somewhat upwards. Costal articulation 8-shaped, the surfaces 



