MYTILID®. 269 
This shell, which is not nacreous inside, is distinguished from 
any of the Mytilide by the form of its ligamental plates and 
muscular impressions ; the hinge-tooth is usually overgrown and 
nearly obliterated by the hinge-margin, as in aged examples of- 
Cardita orbicularis and Cypricardia vellicata. 
Cuanocarpra, Meek and Worthen, 1869. 
Syn.—? Lunulacardium (pt.), Munster. 
Disir.—C. ovata,.M. and W. (cexxx, 34). Fossil; Coal- 
measures, Ills. 
Shell equivalve ? rather thin, very inequilateral, more or less 
oval; beaks small, depressed and nearly terminal; valves 
strongly gaping in front and closed behind ; hinge unkaown, but 
short and without cardinal area; surface with concentric striz, 
crossed on the posterior dorsal region by faintly marked radi- 
ating cost, and on the gaping front by radiating lines and 
costie. 
Mopiomorpua, Hall, 1869. 
Distr—9 sp. Silurian; New York. IM. concentrica, Conr. 
(cxxix, 1D": 
Shell equivalve, very inequilateral, compressed, subovate ; 
beaks small, compressed, within the anterior third of the length; 
surface with rugose or undulating concentric striz, usually coal- 
escing or fasciculating anteriorly ; valves crossed obliquely by a 
more or less distinct sinus, constricting the basal margin; 
anterior end rounded; hinge with a single strong wedge-shaped 
tooth in the left valve and a a corresponding cavity in the right 
valve ; no lateral teeth ; ligament external, attached to the thick- 
ened margin of the shell; ; pallial line entire. 
MODIOLINA, Meek, 1877. J. lata, Meek. Upper Trias; Buena 
Vista Cafon, W. Humboldt Range, United States. Not char- 
acterized. 
PrycuopesMA, Hall and Whitfield, 1872 
Type, P. Knappiana, H. and W. Paleozoic; ‘“ Hydraulic 
Beds,” Louisville, Ky. 
Shell modioloid in form; valves equally convex; hinge with a 
wide ligamental area, the sides of which are sharply : erooved in 
parallel lines, enused by the successive growth of the ligament 
as in Planorbis; the grooves and ridges are slightly arched 
beneath the a apex of the valves, where ‘they t take their origin; 
internal hinge-structure unknown. 
Resembles Modiomorpha in form and surface characters, but 
differs in having a ligamental area. Externally it is unlike 
Cypricardites aaa eutnalat shells, which have a ligamental area 
marked by fine strie parallel to the hinge-line, while these are 
parallel to the margin of the shell. 
