FOSSILS OF THE LOWEE SILURIAN. 



81 



Genus HELICOTOMA Salter. 



Helicotoma sp. f 



The specimens illustrating this species are badly crushed, but there is 

 sufficient form preserved to show that it is allied to H. planulata Salter (Cana- 

 dian Organic Remains, dec. i, p. 14, pi. ii, figs. 5-7) and H. uniangulata Hall 

 (Pal. N. Y., vol. i, p. 179, pi. xxxviii, fig. 8). It is interesting from its occur- 

 rence at an horizon between those of the two species mentioned. The first 

 being from the Calciferous sandstone of New York, and the second from the 

 Trenton limestone of Canada. 



Formation and localities. Upper beds of Pogonip Group. Lone Moun- 

 tain, 18 miles northwest of Eureka, and summit of White Mountain, Eureka 

 District, Nevada. 



Genus MACLUEEA Le Sueur. 



Maclurea annulata, n. sp. 

 Plate xi, figs. 19, 19 a. 



Shell quite small, subdiscoidal, varying from 10 mm to 12 mm in the 

 greatest diameter; spire depressed so as to form a false umbilicus; lower 

 side flattened, the whorls all on one plane. Volutions about two, increas- 

 ing rapidly in size with the increase in growth of the shell; flattened on the 

 under side, rounded or subangular on the lower periphery; the sides rather 

 steep ; transverse section of the outer volution semiovate. 



Surface of the shell somewhat strongly annulated, the annulations on 

 the outer volution having a breadth of about 1.5*. 



Casts of this species show the shell to have been quite thick, and a 

 fragment of the outer volution is near 0.75 mm in thickness. The annulations 

 and the constrictions between are strong, but owing to the thickness of the 

 shell no perceptible traces of them are preserved on the internal cast. 



The cast is more angular on the lower outer margin than on the upper 

 margin; it bears a strong resemblance to M. minima H. and W. (Geol. Expl. 

 Fortieth Par., vol iv, p. 235, pi. i, figs. 17-19), in this condition, but varies 

 in the greater depth of the whorls and the more rapidly increasing size of 

 the inner whorls of M. minima. 

 6 c D w 



