678 PROCEEDINGS OF THE NATIONAL MUSEUM vol.xxiii. 



in the ventral vulve and an elevated ridge or boss in the back portion 

 of the dorsal valve that in one species, H. icJuteavcKt^ appear to have 

 ])een the base of attachment of some portion of the muscular system. 

 The ensemble of the dorsal valve of Bicia is unlike that of either 

 Obolella or Obolus. Bicia, with its thick shell, high area, deep central 

 cavity (heart-shaped cavity of Mickwitz). arrangement of visceral cav- 

 ity, muscle scars, and vascular markings in the ventral valve, is a true 

 Obolus of the (). apoUln is type, but in its more elongate outline, strongly 

 striated surface, and in nearly all details of the dorsal valve it is quite 

 distinct. 



Of the two species now referred to, the genus B. gcnnna has an 

 unusually thick shell and a very marked deposit of shell substance 

 over the visceral area in the ventral valve, the posterior portion of the 

 same area in the dorsal valve, and along its median line. In the second 

 species there is a considerable deposit over the same area with the 

 exception of the median line, where the strong median ridge is absent. 



BICIA GEMMA Billings sp. 



fJhoJcUd (jemma Billings, Can. Xat., 1872, new ser., VI, p. 218, fig. 5, p. 217. 

 OhoMla gemma Walcott, Bull. U. S. Geol. Sur., Xo. 30, 1886, p. 116, pi. x, figs. 



2, 2a-e; Tenth Annual Report U. S. Geol. Sur., 1881, p. 612, pi. lxxi, figs. 



5, 5a-c; pi. lxxii, figs. 2, 2a. 

 OhoMla gemma Hall and Clarke, Pal. X. Y., 1892, VIII, Pt. I, pi. ii, figs. 42-44. 



General form ovate, with ventral valve subacimiinate when the beak 

 is extended or obtusely acuminate when the beak is rounded; dorsal 

 valve oval to subcircular in outline. The convexity of the valves is 

 fairly strong and nearlj^ the same in each where they are embedded in 

 the same matrix. 



Surface of shell marked 1)y numerous slightly irregular concentric 

 strife and lines of growth; by line radiating strite between stronger 

 radiating lines, and on some shells b}^ undulations of strongly devel- 

 oped, elevated, radiating stria?; the radiating stria? are often slightly 

 irregular and interrupted. When the outer sitrface is partiall}" worn 

 away it is smooth, or the shell has a peculiar surface formed by traces 

 of the radiating undulations and stria?. The concentric stria? and lines 

 of growth are shown on the outer surface of the inner laj^ers of the 

 shell and on the interior surface, where rather strong radiating stride 

 are often beautifully shown. 



The shell is usually thick and strong for one so small; those from 

 St. Simon and Troy appear to be calcareous and formed of one solid 

 layer. This is probablv owing to their condition of preservation, as 

 in a weathered specimen lamellre oblique to the outer surface are clearly 

 shown, and indications of lamella on the central and posterior portions 

 that point to the same shell structure as in Obolus and Obolella. 



The ventral valves average about 5 mm. in length; the largest is 



