TRENTON PERIOD. ' 77 



Class GASTEROPODA. 



Subclass DICECA. 



Oeder RHIPHIDOGLOSSA. 



Suborder PODOPBTHALMA. 



Family MACLUR^ID^. 



Genus MAOLUEEA Le Sueur, 1818. 

 Maclurea (?). 



On Amargosa range, at the head of Amargosa Desert, Nevada, a single 

 specimen of Mackirea was found in a mass of rock not in situ. Like the 

 species last noticed, it is too imperfect for specific determination, and is 

 noticed here only because of the value the genus possesses in stratigraph- 

 ical deteiminations. The mass of rock in which it was found had not prob- 

 ably been far removed from its original position, and therefore the presence 

 of Silurian strata in that neighborhood is inferred. 



Family PLEUROTOMAKIIDJE. 



Genus RAPHISTOMA Hall, 1847. 

 Raphistoma trochiscus Meek. 

 Plate IV, fig. 13 a, h, and c. 

 Euomplialus {Raphistoma?) trochiscus Meek, 1870, Proceed. Pbila. Acad. Sci., 61. 



Shell small, sublenticiilar; sphe very slightly convex; suture not dis- 

 tinct; volutions three or fom, flattened-convex both above and below, thin 

 and sharply rounded at the outer side; inner or umbilical side not so 

 sharply rounded as the outer; the inner thud of the upper side impressed 

 to receive the outer portion of the broadly-rounded lower side of the next 

 volution within. The inner edges of the volutions are separate from each 

 other within the umbilicus, but the outer edge of each is so nicely joined 

 upon the next outer one that the convexity of the oiiter part of the upper 

 side of each volution forms a part of the continuous convexity of the spire. 



