168 PALEONTOLOGY. 



one, which ranges nearly vertically; lateral teeth elongated, and only very 

 minutely striated, or very nearly smooth, the posterior one of the left valve 

 being sometimes mainly formed by the beveling of the inflected edge of the 

 valve, instead of standing out at right angles from an erect margin; pallial 

 line with a short, very shallow sinus, immediately under the ovate, shallow, 

 and oblique scar of the posterior adductor muscle (see fig. 6 g) ; anterior 

 muscular impression deeper, slightly smaller than the other, and ranging 

 nearly vertically. 



Length of a large specimen, 1.78 inches; height, 1.56 inches; convex- 

 ity, 1.12 inches. 



As may be seen by our figures, this shell varies greatly in fonm, some 

 specimens being more depressed, with the posterior basal extremity more 

 produced than in others (see fig. 6 &); while others have the umbones more 

 elevated, and the whole shell proportionally shorter, as seen in our fig. 

 6 c. Still other specimens, that seem to be younger examples of this species, 

 are proportionally both more compressed and more depressed, with a more 

 nearly ovate outline, as represented by fig. 6 / It is possible that the 

 latter may belong to a distinct species; but my present impression is that it 

 can hardly be separated from the others, as there seem to be all intermediate 

 gradations of form and size. 



Some varieties of this species look almost exactly like a Lower Lignite 

 Paris Basin species, described by Deshayes under the name Cyrena Forhesi 

 (see plate xxxvii, figs. 24 to 26, Descr. "des An. sans Vert., Bassin, Paris, 1), 

 particularly as seen in a side-view of the exterior. Deshayes' species, 

 however, is a thinner shell, with a weaker hinge, and has its lateral teeth 

 more strongly striated, and the posterior dorsal margins of the valves not 

 near so strongly inflected; while its umbonal slo^Des are less prominently 

 rounded, and it wants the small sinus of the pallial line seen in our spe- 

 cies. In its greater thickness, more trigonal form, and prominent umbonal 

 slopes, our shell agrees more nearly with another species described by 

 Ferussac under the name Cyrena antiqua, and figured by Deshayes (from 

 the Lignite beds of the Paris Basin) on plate xviii of his "Coquilles Fos- 

 siles des- Environs de Paris". The latter species, however, is proportion- 

 ally shorter, with still more elevated beaks, a broader hinge-plate, less 



