NATURAL SCIENCES OF PHILADELPHIA. 411 



Genus GERVILLIA, Defrance. 



Gervillia recta. 



Shell small, rather thin, obliquely elongate-oblong; antero-basal margin 

 nearly straight, or slightly convex in outline; postero-dorsal border ptrallcl to 

 the under margin; anal extremity rounded or subtruncate ; hinge comparatively 

 short, terminating in an acute angle in front, and ranging at an angle of thirty 

 to thirty-five degrees above the oblique longitudinal axis of the shell ; cardinal 

 area scarcely gaping, provided with three small cartilage pits ; beaks small, 

 very oblique, placed about half way between the middle and the anterior ex- 

 tiemity of the hinge; surface smooth, or only marked with fine obscure striaj 

 of growth. Left valve convex, but flattened along the middle, so as to give the 

 shell a cuneate appearance posteriorly, and produce an obscure ridge along 

 the upper and lower margins of the flattened portion ; above the upper of these 

 ridges, the posterior portion of the hinge is strongly compress or subalate. 

 Right valve flat. 



Length, measuring from the posterior extremity obliquely forward to the 

 anterior end of the hinge, 2 inches; breadth, at right angles to the length, 0-66 

 inch; convexity, 0-30 inch; length of hinge, 0-81 inch. 



This little shell is closely allied to a lower green sand species described by 

 Prof. Forbes under the name of Gervilia Unguloides, (Qr. Jour. Geol. Soc. Lon., 

 vol. 1, pi. 3, fig. 9.) So close indeed is the resemblance, that were it not for 

 the fact that our shell holds a position near the upper part of the Cretaceous 

 system, and comes from so widely distant a locality, we should scarcely venture 

 to regard it as a new species. The most important diffeience between it and 

 Forbes' species is in the position of its beaks, those of G. Unguloides being de- 

 scribed as terminal, while in our shell they are placed about half way between 

 the middle and the anterior extremity of the hinge. It is true, D'Orbigny re- 

 fers to G. linguloides, (Pal. Fr. Ter. Cret., Tome 3, pi. 396,) a form which has 

 its beaks not quite terminal, though they are distinctly nearer so than those of 

 our species. In othtT respects D'Orbigny's figures agree almost exactly with 

 the specimens before us, excepting that they represent the shell as being laterally 

 arcuate, while outs is invariably straight. His figures, as well as Prof. Forbes', 

 are also more narrowly rounded at the postero-basal extremity than any of the 

 specimens of the form under consideration. All r.ur specimens are propor- 

 tionably narrower, and more elongate than Prof. Forbes' figures, though not 

 more so than D'Orbigny's. 



Locality and position. Same as last. Very numerous. 



Genus CRENELLA, Brown. 



Crenella elegantula. 



Shell small, very thin and pearly, obliquely ovato-cordate, ventricose ; pos- 

 tero-basal and basal margins rounded ; dorsal border slopine posteriorly with 

 an arcuate outline, and rounding into the anal margin behind; anterior border 

 rounding obliquely backwards into the base; umbonal region of both valves 

 very gibbous; beaks prominent, terminal, pointed, distinctly incurved and di- 

 rected obliquely forward at the extremities; hinge margin smooth ; free border 

 minutely crenulated. Surface (as seen by the aid of a magnifier) beautifully 

 ornamented by extremely fine, regular, closely-arranged, radiating stride, which 

 increase chiefly by bifurcation, and continue of uniform size on all parts of the 

 shell; crossing these are numerous equally fine, but much less distinct, con- 

 centric lines, and occasional stronger marks of growth. 



Length, measuring obliquely forward and upward from the base to the beaks, 

 0-55 inch ; diameter, from base to hinge, measuring at right angles to the 

 greatest length, 0*40 inch ; convexity, 0-37 inch. 



1861.] 



