INVERTEBRATE PALJSONTOLOGT. 143 



slopes forward with a slightly concave outline, while it is convex just behind 

 the beaks, thence declining abruptly; beaks small, rather approximate, 

 located a little behind the middle of the shell. Surface marked by fine lines 

 of growth, and sometimes a i'cw, faint, irregular, stronger, concentric furrows. 



Length, 2.63 inches; height, 1.76 inches; breadth, 1.25 inches. 



Although none of our specimens of this interesting shell are entire, they 

 are quite sufficient to leave little room for doubts respecting its generic rela- 

 tions ; and, when taken together, illustrate very satisfactorily most of its 

 generic and specific characters.* Two of our specimens show the hinge of 

 the left valve, which is provided with one rather strong, obtuse, cardinal 

 tooth, slightly in advance of the point of the beak; while just behind this 

 tooth, and immediately under the beak, there is a distinct pit for the recep- 

 tion of a similar tooth in the other valve. The cardinal edge immediately 

 back of the pit is thickened, so as to form a second, but less prominent, 

 sometimes rather undefined, cardinal tooth. Still farther back, and just at 

 the upper termination of the lanceolate hiatus, formed by the gaping of the 

 valves, the edge projects so as to form a more or less prominent, obtuse, and 

 compressed lateral tooth, or callus. 



The pallial line is very faintly marked, and cannot be distinctly seen 

 along its entire course in any of our specimens, though on one internal cast 

 it may be traced far enough to leave scarcely a doubt of its being simple. 



The anterior muscular impression, as seen in relief on this cast, is very 

 narrow, ovate, and located near the upper margin, a little farther removed 

 from the beaks than from the anterior end of the shell. The posterior mus- 

 cular impression was probably more faintly marked, as it is not visible on 

 this cast. 



In form and general appearance this species resembles Donax securi- 

 formis, Dunker (Palseont. 1, p. 36, pi. vi, fig. 12-14), which, as M. Ter- 

 cjuem has shown, is a true Tancredia (— Hettangia). It differs, however, in 

 being a larger and thicker shell ; and its anterior end is longer and more 

 distinctly rostrate, while the posterior extremity is less angular. It is per- 

 haps quite as nearly related to Tancredia (H.) Dionvillensis and Tancredia 

 (H.) angusta, Terquem (Bull. Soc. Ge"ol. Fr., X, 2e ser., p. 375, pi. 1, fig. 

 1-4, and p. 370, pi. 2, fig. 11-13), but it is a larger and thicker shell than 

 either, and will be readily distinguished by its narrower anterior and rather 



•The liinj;'' of the right, valve, however, is yet unknown. 



