340 OBOLIDZ. 
? Acritis, Volborth, 1869. 
Distr.Silur.; Russia. A. sculpta, Kutorga. 
AcRoTHELE, Linnarson, 1876. 
Distr.—Cambrian; Eur., N. America. A. coriacea, Linnarson. 
Shell horny, consisting of layers, of which the outer one is 
rough, the inner one smooth and shining; ventral shell depressed- 
conical, with perforated apex; from the apex to the hinder 
margin the surface is flattened ; small valve with a curved mar- 
ginal apex arising from two wart-like projections; within having 
two long, diverging muscular impressions in front of the hinge- 
margin, and two small, round ones in the middle, with a median 
ridge between them. 
? Ipnipea, Billings, 1874. 
Distr.—3 sp. Cambrian; Canada, Newfoundland, Sweden. 
I. bella, Billings. 
Resembles Acrotreta, but differs in having a large, convex 
deltidium. 
TREMATIS, Sharpe, 1847. 
Syn.—Orbicella, d’Orbigny, 1847. 
Disir.—Fossil, 14 sp. Lower and Upper Silurian; North 
America and Europe. 7’. terminalis, Emmons (exl, 96-98). 
Valves convex, superficially punctate; dorsal valve with a 
thickened hinge-margin (and three diverging plates, indicated 
on casts.—SHARPE). 
ScuizocrantA, Hall and Whitfield, 1875. 
Distr.—L. Silurian; Ohio. S. filosa, Hall (exl, 5). 
Shell inequivalve, rounded; lower valve attached, posteriorly 
with a deep, triangular sinus, reaching almost to the middle of 
the valve; upper valve convex with posterior beak and six 
internal muscular impressions. 
SrpHonoTReTA, Verneuil, 1842. 
Etym.—Siphon, a tube; tretos, perforated. 
Syn.—? Mesotreta, Kutorga, 1848. 
Distr.—Fossil, 9 sp. Lower and Upper Silurian; Britain, 
Bohemia, Russia. S. verrucosa, Vern. (cxl, 6-8). 
Shell oval, biconvex, slightly beaked, conspicuously punctate, 
or spiny; beak perforated by a tubular foramen; hinge-margins 
thickened; ventral valve with four close adductor sears sur- 
rounding the foramen. The spines are tubular, and open into 
the interior of the shell by prominent orifices.—CARPENTER.  S. 
anglica, Morris, has moniliform spines. 
