OBOLID^. 339 



in the ventral valve ; posterior adductor impressions separate ; 

 anterior pair subcentral ; impressions of adjusters lateral. 



Obolella, Billings, 1861. 



Eym. — Diminutive of Obolus. 



Syn. — ? Kej'^serlingia, Pander, 1861. 



Distr. — 12 sp. Cambrian, Lower Silurian; United States, 

 Canada, England, Spain. 0. chromatica, Billings (cxl, 99, 100). 



" Shell ovate, circular or subquadrate, convex or plano- 

 convex ; ventral valve with a false area, which is sometimes 

 minute, and usually grooved for the passage of the peduncle ; 

 dorsal valve either with or without an area ; muscular scars in 

 the ventral valve four ; one pair in front of the beak near the 

 middle, or in the upper half of the shell, and others situated 

 one on each side near the cardinal edge ; shell calcareous ; sur- 

 face concentrically striated, sometimes with thin, extended, 

 lamellose edges. 



" In general form these small shells somewhat resemble 

 Obolus, but the arrangement of the muscular impressions is 

 different. In Obolus the two central scars have their smaller 

 extremities directed downwards, converging towai'ds each other ; 

 but in this genus the arrangement is exactly the reverse." — 

 Billings. 



KUTORGiNA, Billings, 1861. Hinge-line straight, nearly as wide 

 as the shell, sides nearly straight, meeting the cardinal border 

 at an obtuse angle ; neural valve with an area and foramen ; a 

 pair of subcentral, oval muscular impressions, but no vestiges 

 of lateral scars ; externally radiately striate. Cambrian ; 

 Canada. 0. cingulata^ Billings (cxl, 1, 2). 



MONOBOLiNA, Salter, 1865. Shell resembling Obolus; broad, 

 external surface radiately striated ; muscular scars united 

 closely along the central line. Silur. ; England. 0. 2)lumbea, 

 Salter. 



ScHMiDTiA, Volborth, 1869. 



Syn. — Dicellomus, Hall? 1811. 



Distr. — Silur., Devon.; Russia, N. America. 



Shell very small, long-oval, shining ; large valve swollen, with 

 pointed beak, and grooved area ; within two deep muscular 

 impressions ; in the small valve a raised ledge between the 

 impressions. 



Leptobolus, Hall, 1811. 



Distr. — 3 sp. Silurian ; N. America. L. lepis, Hall (cxl, 3, 4). 



Shell small, thin, swollen, oval ; large valve with a short beak, 

 and deepl}^, broadly grooved area; within a two-pai-ted, some- 

 what raised muscle-plate ; small valve slightly thickened dor- 

 , with an internal, three-parted, diverging, muscular ' 

 ession. 



