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 imner and lateral margins, and do not extend 
to the outer layer of the shell, with which they are, apparently, unconnected. 
CRASSATELLA. JZamarck, 1799. 
TELLINA (sp.), Solander. 
Macrra (sp.), Brug. 
Prycuomya (sp.), Agass. 
Papua, Roissy, 1805. 
Goutpra, C. B. Adams. 
Generic Character —Shell thick and strong, transverse, occasionally sub-orbicular or 
triangular, closed ; inequilateral, smooth, or covered with raised strize, 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 As¢arte, 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 fimbriz 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 striz. 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 pluméea, 
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, 
29 
aw 
