BIVALVIA. 161 



teeth. The crenulations on the margin are broad on each side, and diverge in nearly 

 the same direction. They are confined to the inner and lateral margins, and do not extend 

 to the outer layer of the shell, with which they are, apparently, unconnected. 



CRASS A TELL A. Lamarck, 1799. 



Tellina (sp.), Solander. 

 Mactra (sp.), Briig. 

 Ptychomya (sp.), Agass. 

 Paphia, Roissy, 1805. 

 GouLDiA, C.B.Adams. . 



Generic Character. — Shell thick and strong, transverse, occasionally sub-orbicular or 

 triangular, closed ; inequilateral, smooth, or covered with raised striae, imbrications, or ridges 

 (thickened lines of growth) ; lunule distinct ; hinge with two cardinal diverging teeth. 

 Connexus cartilaginous, placed in a triangular fossette ; lateral teeth often obsolete ; 

 adductor muscles deeply impressed ; pedal muscle-mark distinct ; palleal impression entire. 



Animal with the mantle lobes open or only united by the branchial septum ; foot of 

 moderate size, compressed and grooved. 



This genus appears to be nearly related to Astarte, and, in like manner, most of the 

 species have the inner margins of the valves covered with crenulations, but I cannot say 

 that such differences constitute a mark of distinction between the young and aged. These 

 crenulations are confined to the inner portion of the shell, and they appear to be smaller 

 and finer in the young, as if the fimbrise of the mantle were of the same number at all 

 ages. Beneath the surface on some of the species (perhaps all) the shell is covered with 

 radiating striae. I am not able to say if the crenulations upon the margin be the outcrop 

 of these rays, but I presume they are produced by a more or less fimbriated margin of 

 the mantle. 



The shells of this genus are not only thick and strong, but they are capable of being 

 very firmly closed by powerful adductor muscles, which leave a deep impression in the 

 interior, and they are furthermore secured from lateral motion by prominent denticles, 

 which are sometimes ridged at right angles to their position, corresponding in that respect 

 to some species of Astarte. 



There is great variation in magnitude in the species of this genus. Crassatella plumhea, 

 a shell common to the Paris Basin, but which, I believe, has not yet been found in this 

 country, is of great thickness ; a single valve weighs half a pound. Some American 

 fossil species have a length of six inches ; others, on the contrary, are very diminutive. 

 The genus is abundant in species in the Lower Tertiaries of this country, but it has not, 



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