340 OBOLID^. 



? AcRiTis, Volborth, 1869. 

 Distr. — Silur. ; Russia. A. scidjjta, Kutorga. 



AcEOTHELE, Liimarsoii, 1816. 



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. 



? Iphidea, Billings, 1874. 



Distr. — 3 sp. Cambrian ; Canada, Newfoundland, Sweden. 

 I. bella, Billings. 



Resembles Acrotreta, but differs in having a large, convex 

 deltidium. 



Trematis, Sharpe, 1841. 



Syn. — Orbicella, d'Orbign}^, 1841. 



Distr. — Fossil, 14 sp. Lower and Upper Silurian ; North 

 America and Europe. T. terminalis, Emmons (cxl, 96-98 \ 



Valves convex, superficially punctate ; dorsal valve with a 

 thickened hinge-margin (and three diverging plates, indicated 

 on casts. — Sharpe). 



ScHizocRANiA, Hall and Whitfield, 1815. 



Distr. — L. Silurian ; Ohio. S. filosa.^ Hall (cxl, 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. 



SiPHONOTRETA, Yerueuil, 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 spinjr ; beak perforated by a tubular foramen ; hinge-margins 

 thickened ; ventral valve with four close adductor scars 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. 



