MIOCENE MOLLUSCA AND rKUSTAOEA. 91 



cluced, compressed, much retlexed; the end acutely rounded." In his 

 observations on it he says tlie iigure there given " is only approximate to 

 the true outline, as tlie shell is* slightly crushed." It is. true the figure is 

 only approximate to the f>utline of the specimen, but the crushing has been 

 so very slight tliat it forms but little excuse for the imperfection of the outline. 

 There is but a single (right) vahe of the species for the basis of the above 

 descriptions, and no specimen of it has turned up in other collections. I 

 can see no reason for the removal of the species from the genus Saxicava to 

 Tlu-acia, as the shell does not present the features requisite to the latter 

 genus, being entirely destitute of the projecting crescentic ossicle character- 

 istic of it. It appears, however, to possess the hinge features of Saxicava 

 so far as can be seen from the specimen, which has been cleared out around 

 the hinge for the determination of these features. I can, however, find no 

 evidence whatever of the two small cardinal teeth in the right valve spoken 

 of in Mr. Coni-ad's first description. There may possibly have been one 

 very minute tooth, but if so it has been destroyed, and what may have been 

 mistaken for a sec-ond is only a fracture and slight displacement of the 

 shell. The valve mav be characterized as follows: 



Shell small, only the right valve known, wliich is moderately convex 

 and transversely ovate in outline; the beak, which is proportionally large, 

 being situated at about one-third of the length of the valve from the anterior 

 end. Anterior end rounded below and obliquely sloping from the beak to 

 near the middle of the height on the upper side. Posterior end narrower, 

 rounded below and sloping above, a short part of the cardinal border near 

 the beak being nearly straight and parallel to the basal margin. Base very 

 broadly rounded. Body of the valve ^ibbose from the beak to the base for 

 the anterior half, but recurved posteriorly. Surfiice marked with distinct 

 concentric lines. The liinge is narrow and obscure, l)ut showe a short nar- 

 row ligameutal area of attachment posterior to the apex of the shell, and a 

 depressed false groove anterior to the beak. 



The hinge of this valve is an exact counterpart of that shown on most 

 of the distinctly marked examples of Saxicava rugosa, as seen in both recent 

 and post-Pliocene fossil specimens, and shows a ver}- close generic aftinity 

 with that species. The internal characters of the other portions of the valve 



