220 



merous radii ; beaks a little prominent in consequence of tlie con- 

 cave curvature of the 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 naiTowed 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. 

 Ulacina, Lam., and much less broad than C. gigantea, Gm., of 

 Florida. The specimen is very much 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 5 hinge having one or two teeth on each valve, or upon 

 one of the valves only ; palleal impression profoundly sinuous an- 

 teriorly. 



Animal with two syjjhons, 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- 

 niental 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 Litliaphagu, 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 Venrnqm, 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. 

 Veneriipis approaches nearer to Vc7ms in the number of its teeth, 

 which, however, are not divaricated as in the latter genus. 



