FROM THE TERTIARY OF VIRGINIA. 13 



This shell is closely allied to the preceding, but the differences in outline, hinge and 

 sculpture separate them. 



FAMILY LITHOPHAGDXE. 



GENUS PETRICOLA. 

 P. COMPRESSA, PL. 34. FIG. 15. 



P. testa valdc transversd, valdc in&quilaterali, sub-ellipticd, compressd, postice sub-truncatd. antice rotundatd, tenui, 

 striatd postice et antice; striis minimis, concentricis; margins basalt recto; margine dorsali recto; natibus sub-acutis, 

 prominenlibus; dentibus duobus, parvis, divergentibus. 



Shell very transverse, very inequilateral, sub-elliptical, compressed, posteriorly sub-truncate, anteriorly rounded, 

 thin, striate posteriorly and anteriorly; stria; very small, concentric; basal margin straight; dorsal margin straight; 

 beaks somewhat acute, prominent; teeth two, small, divergent. 



Diam. .10. Length .12. Breadth .25 of an inch. 



Remarks. The concentric lines of growth are very small. They are visible on the 

 anterior and posterior portions of the shell. On the central part they are obsolete, 

 leaving it smooth. The teeth are small and unusually divergent for a Petricola. 



This shell is smaller and more compressed than any of our described species. In 

 form it much resembles Saxicava vaginoides, Desh., but it is more transverse and differs 

 in the hinge. 



FAMILY NYMPHID.E. 



GENUS PSAMMOCOLA, (BLAINVILLE.) 

 P. LUCINOIDES, PL. 34. FIG. 16. 



P. testa sub-orbiculari, pene cequilaterali, postice et antice rotundatd, convexd, tenui, striatd; striis concentricis, 

 minimis; natibus prominentibus, sub-rectis; valvd sinislrd dentibus duobus cardinalibus, quorum unus bijidus, late- 

 raiibus nullis. 



Shell suborbicular, almost equilateral, rounded anteriorly and posteriorly, convex, thin, striate; striae concentric, 

 very small; beaks prominent, nearly straight; left valve with two cardinal teeth, one of which is bifid, and no late- 

 ral ones. 



Diam. .15. Length .31. Breadth .35 of an infth. 



Remarks. This shell is unusually lenticular for a Psamnoocola. The concentric 

 striae are numerous and extremely small. There are, however, three or four larger ones, 

 nearly equidistant, over the surface of the shell. 



The general resemblance which this shell bears to some species of the genus Lucina, 

 will readily distinguish it from its congeners. 



I have adopted this genus on the authority of Sowerby, who says that in it Blainville 

 unites the Psammobia and Psammoteea of Lamarck, considering the difference between 

 these genera as artificial and accidental. 



P. REGIA, PL. 34. FIG. 17. 



P. testa transversa, ovato-quadratd, inaequilalerali, postice hiante, antice et postice subtruncatd, valde inflata, 

 crassissimd, sulcatd; sulcis radiantibus, crebris, aqualibus, latis; margine basali pene recto; margine dorsali sub-cur- 



