195 
TRIGONIA. 
Gen. Char. An equal-valved inequilateral trigonal 
bivalve, hinge with two diverging elongated 
compressed teeth, transversely grooved on each 
side, and fitting into grooved cavities in the 
opposite valve. 
Ihe trigonal form, although admitted as a Generic Cha- 
racter, is often very obscure. The anterior side of the 
shells of this Genus is generally separated from the rest by 
a prominent ridge, and the whole surface is more or less 
ornamented. In Trigonia margaritacea, of Lamarck, a 
recent shell which led to the discovery of the nature of the 
hinge of this Genus, it is said that there are two muscular 
impressions, but in the fossil species 1 have not been able to 
trace more than one : ought they not therefore to be con- 
sidered as distinct Genera? The difference is of conse- 
quence to those who doubt the identity of recent with fossil 
organization. 
TRIGONIA costata. 
TAB. LXXXY. 
Spec. Char. Triangular, with transverse smooth 
ribs; anterior side marked with many small 
and three large prominent longitudinal c min- 
iated ridges. 
Syn. Curvirostra non rugosa, &c. Luid. Lithoph . 
p. 36. T. 9. F 714. 
Trigonia costata. Park. Org. Rem . 3. T. 12. F. 4 9 
Ihe posterior angle is very obtuse ; the anterior side large, 
with an obtusely three-angled margin, corresponding to the 
three larger longitudinal ribs, before the first of which the 
