188 PALEONTOLOGY OF THE EUREKA DISTRICT. 



This species is closely related to several described forms, but differs 

 chiefly in having an unusually narrow umbilicus. 



Formation and locality. Central portion of the Devonian limestone; 

 Newark Mountain, Eureka District, Nevada. 



Genus PLATYSCHISMA McCoy. 



Platyschisma ? McCoyi, n. sp. 



Plate xvii, figs. 1, la-c. 



Shell subdiscoidal in its younger stages of growth, becoming more or 

 less trochiform in the adult ; spire moderately elevated. Volutions five or 

 six, expanding gradually from the apex to the aperture; periphery rounded, 

 acute; upper surface with a distinct depression just above the periphery, that 

 increases in width with the enlargement of the volutions so as to give, in 

 the adult, a concavity to the surface of the last volution; under side of 

 volutions slightly convex; transverse section subrhombic. Aperture irregu- 

 larly rounded with a sinuosity in the upper margin. 



Surface marked by lines of growth that curve back to a narrow line 

 or band, and then forward to the periphery of the whorl; on the last whorl 

 these lines have a broad retral curve from the periphery to the umbilicus. 



This species differs from the generic type of Platyschisma in having a 

 closed umbilicus, and thick shell, but in the typical species the umbilicus 

 is very small, and M. Chenu has placed a thick shell, P. applanatus, 12 as 

 representing the genus. In other respects the generic characters are those 

 of Platyschisma, the narrow line formed by the meeting of the strigs not 

 forming a true band as in Pleurotomaria. 



Formation and locality. Central portion of the Devonian limestone ; 

 Newark Mountain, Eureka District, Nevada. 



Platyschisma? ambiguum, n. sp. 

 Plate xvii, figs. 3, 3 a. 



Shell subdiscoidal, spire short, obtuse. Volutions about five, angular 

 and closely united at the suture, forming a slightly convex, unbroken slope 



"Mannal de Conch yliologie, p. 235, fig. 1382, 1859. 



