FROM THE TERTIARY OF VIRGINIA. 243 



Shell obliquely triangular, inequilateral, posteriorly and anteriorly angulated, inflated, rather thick, striate: con- 

 centric striae very small; longitudinal striae, few, almost obsolete; umbonial slope steep; basal margin curved; dorsal 

 margin acutely angular in the middle, posteriorly and anteriorly straight; teeth small; fosset small, sub-triangular; 

 internal margin crenulate. 



Diam .17. Length .18. Breadth .20 of an inch. 



Remarks. The longitudinal stri are only visible in a strong light, with a powerful 

 microscope. The concentric lines of growth are also small, with the exception of two or 

 three larger ones. 



This may prove to be only a variety of N. obliqua, Say, but it differs from all my 

 specimens of that shell in being more inequilateral and inflated, in the form of the fosset, 

 and in the straight series of teeth. 



N. DIAPHANA, PL. 34. FIG. 26. 



N. testa transvcrsd, incequilaterali, sub-ellipticd, postice et antice rotundatd, injiatd } diaphand, tenui, lavi, politd, intus mar- 

 garitaced- clivo umboniali sub planulato; margine basali curvato; margins dorsali curvato; natibus prominentibus; dentibus valde 

 arcuatis, anticis quinque, posticis duodecim; cochlea rhoinboided } obliqua; margine interno crcnulato. 



Shell transverse, inequilateral, sub-elliptic, rounded anteriorly and posteriorly, inflated, diaphanous, thin, smooth, 

 polished^ pearly within; umbonial slope somewhat flattened ; basal margin curved ; dorsal margin curved ; beaks pro- 

 minent; teeth very arcuate, five anterior, twelve posterior; fosset rhomboidal, oblique; internal margin crenulated. 



Diam. .04. Length .11. Breadth .15 of an inch. 



Remarks. This little shell, in common with the preceding, has some resemblance to 

 the N. obliqua, Say, but it may be distinguished by its elliptical shape, thin and diapha- 

 nous substance, smooth surface, and rhomboidal fosset. The teeth are also more arcuate, 

 and there is a greater disparity between the posterior and anterior series. 



In outline, this species is closely allied to the N. antiqua, Mighels and Adams, from 

 the Post-Pliocene of Massachusetts. 



N. ^QUILATERA, PL. 34. FIG. 27. 



N. testa longitudinaliier-ellipticd, fequilaterali, postice et antice sub-curvatd, inflatd } crassd, punctato-costatd; 

 costis radiantibus, vel punctiferis vel cancellatis; margine basali valde curvato; margine dorsali angulato; natibus 

 prominulis, incurvis, rectis; dentibus parvis, rectis, anticis tribus, posticis octo; cochlea pared, profundd; margine 

 interno crenulato. 



Shell longitudinally elliptical, equilateral, somewhat curved posteriorly and anteriorly, inflated, thick, punctately 

 costate; costse small, radiating, punctiferous or cancellate; basal margin very much curved ; dorsal margin angulate; 

 beaks somewhat prominent, incurved, straight; teeth small, straight, three anterior, eight posterior; fosset small, 

 deep; margin crenulate within. 



Diam. .08. Breadth .10. Length .12 of an inch. 



Remarks. The ribs are small, sometimes divaricate, and punctulate. They occa- 

 sionally have the appearance of being cancellate. The shorter series of teeth is imme- 

 diately under the beaks and adjoining the fosset, from which the longer series is removed 

 by a smooth intervening space. This character, and the longitudinal equilateral shape 

 seem to remove it from the genus Nucula, but they can hardly be said to amount to 

 generic distinctions. 



This shell bears a general resemblance to the Myoparo costatus, Lea, but its shape 

 does not admit it into that genus. 



