OBOLIDZ. 339 
in the ventral valve; posterior adductor impressions separate ; 
anterior pair subeentral ; ; impressions of adjusters lateral. 
OsoLeELLA, Billings, 1861. 
EHym.—Diminutive of Obolus. 
Syn.—? Keyserlingia, Pander, 1861. 
Distr—12 sp. Cambrian, Lower Silurian; United States, 
‘Canada, England, Spain. 0. chromatica, Billings (exl, 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. 
ped bal eeneral 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 towards each DEnER: 
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. O. cingulata, Billings (exl, 1, 2 
MONOBOLINA, Salter, 1865. Shell resembling Obolus; broad, 
external surface radiately striated; muscular scars united 
closely along the central line. Silur.; England. O. plumbea, 
Salter. 
Scumrptia, Volborth, 1869. 
Syn.—Dicellomus, Hall? 1871. 
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. 
Leproxsowus, Hall, 1871. 
Distr.—3 sp. Silurian; N. America. JZ. lepis, Hall (exl, 3, 4). 
Shell small, thin, swollen, oval; large valve with a short beak, 
and deeply, broadly erooved area ; within a two- parted, some- 
what raised muscle-plate ; small valve slightly thickened dor- 
with an internal, three-parted, diverging, muscular 
ssion. 
