THE TERTIARY FORMATIONS OF VIRGINIA. 373 



minent ; beaks small, slightly incurved, and not distant. Length equal 

 to the breadth, three inches. 



Locality, western part of the peninsula of the Potomac and Eappa- 

 hannock, Virginia, in the eocene. 



Description. — Alternate longitudinal striae, very obscure and deli- 

 cate, divide many of the costaa along the centre, and throw them, espe- 

 cially next the anterior side, into pairs. The beak has four-sevenths 

 of the length of the hinge on its posterior side. This species may be 

 known from the 0. incerta of Deshayes by its much greater size, its less 

 quadrangular form, by the greater number of its lateral teeth, and the 

 less incurvation of its beaks. It is not less readily distinguished from 

 C. gigantea, a species prevailing in the same beds with it, by its less 

 width, compared to its height, by the less obliquity and greater length 

 of its posterior margin, by the beaks being less remote and less in- 

 curved, and by the shell being smaller, but materially thicker, and 

 more inflated near the base. A prominent feature is the great infla- 

 tion of the valves, especially towards their base. The hinge is well 

 marked by from four to five lateral teeth, next the posterior side, and 

 from three to four next the anterior, all being slightly curved, striated 

 by deep irregular grooves on their sides, and of nearly equal obliquity. 

 The central longitudinal teeth are numerous, irregular, and slightly 

 oblique. The area of the ligament is nearly a segment of a circle, the 

 straight hinge line being the chord; its surface is marked by about six 

 deep, rather waved grooves. The right valve is the largest, overlap- 

 ping the left on the lower margin, which is moderately crenulated in 

 both. In the cabinet of the Academy of Natural Sciences of Philadel- 

 phia there are, besides three large casts of C. gigantea, two apparently 

 of the present species, somewhat larger than the shell now described. 

 These latter, I have satisfied myself, belong to C. onochela, as an inter- 

 nal cast of this made in wax is precisely like them, though very dif- 

 ferent from the casts of C. gigantea. 



Cucullea transversa. Plate XXIX., fig. 1 . 

 Specific character. — Shell subovate, subtrapeziform, oblong, oblique, 

 inequilateral, inequivalve; longitudinal strijE numerous and delicate, 



