FOSSILS OF THE LOWEE SILURIAN. 235 



GASTEROPODA. 

 Genus RAPHISTOMA Hall. 



EHAPHISTOMA acuta I). S^). 

 Plate I, tigs. 20-22. 



Shell small to medium size, lenticular in form, and sharply acute on 

 the periphery; volutions three to three and a half, moderately increasing in 

 size with the growth of the shell; suture close. Upper surface of the volu- 

 tions less convex than the lower, very slightly rounded below the suture- 

 line, and a little concave just within the margin, giving an acutely angular 

 edge. Lower surface of the volution once and a half as deep as the upper 

 part, measuring from the angle ; subangular or sharply rounded at the edge 

 of the umbilicus, which is nearly one-third as wide as the entire diameter 

 of the shell, showing all of the inner whorls when free from rock. Aperture 

 triangular, wider than high, acute on the outer edge, the widest part being 

 above the middle. Surface of the shell not observed, the specimens being 

 either internal casts or having partially-exfoliated surfaces. 



The species bears considerable resemblance to B. lenticularis Sow. 

 from the Trenton limestone, but is more depressed, and the volutions are 

 more angular on the lower side. In this latter feature, it corresponds more 

 nearly with the forms of the genus, occurring in the Chazy and Calciferous 

 formations than with those from the Trenton group, and bears a striking 

 resemblance to some forms of E. staminea Hall, but has probably not had 

 the strong surface-markings of that species. It differs from Euomplialus f 

 rotuliformis and E. 9 trocMscus Meek in having a smaller number and more 

 rapidly-increasing volutions. 



Formation and locality. — In limestone of Lower Silurian age, probably 



Chazy, at Ute Peak, Wahsatch Range, Utah. Collected by Arnold Hague, 



esq. 



Genus MACLUREA Lesueur. 



Maclurea minhma n. sp. 



Plate I, figs. 17-19. 



Shell quite small, subdiscoidal, the largest individuals not measuring 

 more than four-tenths of an inch in their greatest diameter. Spire depressed, 



