195 
TRIGONIA. 
Gen. Cuan. 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. 
ae a 
Tue 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. Cuar. ‘Triangular, with transverse smooth 
ribs; anterior side marked with many small 
and three large prominent longitudinal crenu- 
lated ridges. 
Syn. Curvirostra non rugosa, &c. Zuid. Lithoph. 
p. 36. T.9. F. 714. | 
Trigonia costata. Park. Org. Rem. 3.7.12. F. 4. 
re Fn 
\ 
Tar 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 
KEUE MVE. 
