360 SYSTEMATIC PALEONTOLOGY 



subequilateral, slightly gaping at both extremities, inequivalve, the right 

 valve being the more convex, its whole margin projecting considerably 

 beyond that of the left; beaks protuberant, large and cordiform, inclining 

 a little backwards, the summit of the right one excavated or emarginate 

 to receive the opposing one; incremental striae numerous and distinct, 

 occasionally forming feeble concentric ridges ; the anterior portion of the 

 shell is regularly rounded and its superior margins very thin; the pos- 

 terior extremity is rather narrower and somewhat truncated, with an 

 obtuse carination extending obliquely from the beaks to the angle of the 

 basal and posterior margins ; between this carination and the superior and 

 posterior margins the shell is slightly compressed. The basal margin is 

 sinuous, curving outwardly in its central portion, correspondent to the 

 most convex part of the shell. Ligament externally very prominent, and 

 prolonged in a thin membrane the whole length of the corselet which is 

 strongly marked and extends from the beaks to the extremity ; the internal 

 portion of the ligament is attached to a strong, thick nymphal callosity, 

 projecting obliquely along the cardinal edge in each valve, wider toward 

 the beaks and having its surface but slightly hollowed. Hinge destitute 

 of a cardinal ossiculum. External color a pale, ashy-white surface cov- 

 ered with a thin, light, cinereous epidermis, strongly adherent and form- 

 ing numerous irregular, minute corrugations at the extremities, especially 

 on the posterior one, but not shagreened as in T. corbuloides. Interior 

 color a chalky white, not glassy, but somewhat inclining to nacre near the 

 cardinal edge. Muscular impressions tolerably large, remote, the ante- 

 rior narrow, elongated, contracted and tapering to a point towards the 

 hinge margin; the posterior subtriangular or pyriform; pallial impres- 

 sion very superficial, like the others, with a profound, subangular excava- 

 tion posteriorly. , 



" Length two and eigh teen-twentieths, height two and four-twentieths, 

 diameter one and six-twentieths inches." Couthouy, 1839. 



The fossil shell seems usually to be larger than Couthouy's living ones. 

 Although often abundant, all specimens the writer has seen have been 

 more or less broken and flattened. Because of this distortion their 

 exact shape is diflficult to determine and the writer prefers to retain until 

 more perfect material is obtainable the name conradi. When such 



