FOSSILS OF THE DEVONIAN. 



189 



from the apex to the acute periphery ; transverse section subtrigonal ; upper 

 surface with a slight depression below the center; under surface slightly 

 convex. Umbilicus closed. Aperture nearly round, with the inner lip 

 reflected. 



Surface transversely marked by lines of growth that have a retral 

 curve on the central portion of the upper surface of the volution. 



This species is doubtfully referred to Platyschisma, owing to its having 

 a reflected inner lip and closed umbilicus. The latter, however, was prob- 

 ably open in the earlier stages of growth, having been closed by the outer 

 volutions. The general aspect of the species is much like that of several 

 forms of Pleurotomaria occurring in the Silurian formations, but the 

 absence of a definite mesial band and the presence of the broad sinuosity 

 in the upper portion of the aperture, shown by the retral curve of the 

 lines of growth, distinguishes it and places its generic relations with Platy- 

 schisma or an allied genus. 



Formation and locality. Upper horizon of the Devonian limestone, The 

 Gate, Eureka District, Nevada. 



Genus CALLONEMA Hall. 



Callonema occidentalis, n. sp. 



Plate xvi, figs. 3, 3 a. 



Shell subglobose, depressed. Spire slightly elevated, consisting of 

 about four rounded volutions very regularly increasing in size from the 

 apex to the aperture; suture slightly depressed and marking the periphery 

 of the preceding volutions. 



Surface exfoliated. The cast shows a few obscure transverse strise that 

 curve gently backward from the suture towards the periphery. 



The general aspect of this species is similar to that of Callonema imitator 

 H. and W. (Hall, Pal. N. Y., vol. v, pt. 2, p. 53, pi. xiv, figs. Itf, 17) of the 

 Upper Heiderberg Group of the State of Indiana. It differs in having a 

 more depressed spire, and the volutions expanding a little more rapidly. 



Formation and locality. Lower horizon of the Devonian limestone, 

 Comb's Peak, Eureka District, Nevada. 



