50 . SUPPLEMENT TO GREAT OOLITE MOLLUSCA. 
numerous, smooth, slender, nearly horizontal, and gracefully curved; the anterior border 
is rounded ; the area is lengthened, narrow, and slightly excavated, having three carine, 
of which the marginal and inner carime are conspicuous, curved, and crenulated ; the 
median carina is small; the spaces between the carinze have longitudinal, delicate, closely 
arranged, granulated little costee; the post-ligamental lanceolate space has a very 
delicately reticulated surface. 
A transversely lengthened, large, and gracefully curved form, much less convex than 
T. costata, with more depressed umbones, and having the anterior side greatly more 
rounded and produced. ‘The costated portion of the shell is very large; the coste are 
numerous, not much elevated; their direction is nearly horizontal, excepting near to the 
apex, where they have an elegant sigmoidal curvature. The length upon the marginal 
caring is one fourth greater than the opposite measurement; the diameter through both 
the valves is somewhat less than half the height. The area is very narrow and length- 
ened, the ornamentation of its surface is minute and delicately sculptured, the general 
figure is depressed, which, together with the small prominence of the umbones, the 
excavated posterior side, and the large, rounded anterior side, will, in the aggregate, serve 
to distinguish it from other allied forms. 
Geological Position and Localities. It is moderately abundant in the Cornbrash of 
the coast of Yorkshire. The foreign localities quoted by D’Orbigny are Luc (Calvados), 
Vezelay (Yonne), Grange-Henry, near Nantua. 
TRigon1a compra, Lyc. Tab. XL, fig. 1. 
Testa ovato-trigona, subdepressa, umbonibus obtusis, depressis, /atere antico brevi, 
rotunda, postico producto ; area planata, carinis tribus delicatissimis et striis transversis 
tenuissimis instructis, costis (circa 12) posticé nodulosis, interruptis, antice rugis obliquis 
instructis. 
Shell ovately trigonal, rather depressed ; umbones obtuse, depressed, not recurved ; 
anterior side short, rounded, posterior side more produced, its border straight, sloping 
obliquely downwards; area flattened, with three very delicate, tuberculated carine, and 
fine transverse striations, lanceolate ; post-ligamental space narrow, lengthened, and smooth. 
The other portion of the surface has about twelve rows of coste, which become large, hori- 
zontal, nodulose, interrupted varices posteriorly, and form small, oblique, rather imperfectly 
tuberculated, but continuous coste anteriorly, so that all the costa reach the anterior 
border in an oblique rather than in a concentric direction. 
The diagnostic characters are not very strongly marked, but im the aggregate are 
sufficiently distinctive. ‘The delicately ornamented area separates it from 7. Moretonis, 
Mor. and Lyc., and the posterior, interrupted varices from 7. impressa, Sow. From 7: 
Goldfussii, Ag., it is distinguished by the smaller oblique coste, more especially of their 
