220 
merous radii ; beaks a little prominent in consequence of tbe con- 
cave curvature of tbe posterior dorsal margin ; lunule rather large, 
impressed, distinct, oblong-cordate ; posterior side prominent ; an- 
terior dorsal margin slightly arquated depressed towards the beaks, 
obtusely carinated on the submargin ; anterior tip narrowed and 
rounded ; within, margin simple ; fosset of the posterior tooth 
simple. 
Ohs. This fossil shell was sent to me by the late Mr. Stephen 
Elliott who informed me that it was found on the banks of the 
Santee river, below the confluence of the Congaree and Wateree 
rivers in South Carolina. It is proportionally broader than C. 
lilacina, Lam., and much less broad than C. gigantea^ G^m., of 
Florida. The specimen is very mueh thickened within on the 
inner side of the palleal impression, which is deeply sinuous an- 
teriorly. Plate 59. 
Petricola. — Shell bivalve, transverse, subtrigonate, or oblong, 
inequilateral, rounded behind, anteriorly narrowed and a little 
gaping ) hinge having one or two teeth on each valve, or upon 
one of the valves only ; palleal impression profoundly sinuous an- 
Animal with two syphons, divided only at tip, unequal in 
length and diameter, contractile within the shell ; mantle thicker 
on the margin and united, with a very small opening for the rudi- 
mental conic foot; mantle very small, transverse, chiefly con- 
cealed by two lips, which are extended laterally into small triangu- 
lar palpi. 
Ohs. Lamarck in his last work, united Rupellaria of Bellevue 
to his Petricola, in his family Lithopliaga, which, as its name 
implies, contains those genera of shells, which are destitute of 
accessory pieces and have the remarkable property of penetrating 
calcareous rocks and thus establishing for themselves a permanent 
and secure dwelling. This family consists of three genera, Saxi- 
cava, Petricola and Venerupis, which are closely allied. The 
hinge of the former is destitute of teeth, or in some instances has 
only obsolete tubercles, and the syphons of the animal are longer 
than in the present genus, and united to the extremity in one fleshy 
envelope, not retractile within the shell, but always in part exposed. 
Verier upis approaches nearer to Venus in the number of its teeth, 
which, however, are not divaricated as in the latter genus. 
