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 carinae, 

 of which the marginal and inner carinas are conspicuous, curved, and crenulated ; the 

 median carina is small; the spaces between the carinse have longitudinal, delicate, closely 

 arranged, granulated little costse ; the post-liganiental 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 costse 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 

 carinse 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 disthiguish 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. 



Trigonia eoMPTA, In/c. Tab. XL, fig. 1. 



Testa ovato-trigona, siibdept'essa, umbonibus obtusis, dejyressis, latere antico brevi, 

 rotunda, postico prodncto ; area planata, carinis tribus delicatissimis et striis transversis 

 tenuissimis instructis, costis {circa 12} postice nodulosis, interruptis, antice rugis obliquis 

 instrifctis. 



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 carinse, and 

 fine transverse striations, lanceolate ; post-ligamental space narrow, lengthened, and smooth. 

 The other portion of the surface has about twelve rows of costaj, Avhich become large, hori- 

 zontal, nodulose, interrupted varices posteriorly, and form small, oblique, rather imperfectly 

 tuberculated, but continuous costse anteriorly, so that all the costae reach the anterior 

 border in an oblique rather than in a concentric direction. 



The diagnostic characters are not very strongly marked, but in the aggregate are 

 sufficiently distinctive. The delicately ornamented area separates it from T. Moretonis, 

 Mor. and Lye, and the posterior, interrupted varices from T. impressa, Sow. From T. 

 Gold/ussii, Ag., it is distinguished by the smaller oblique costae, more especially of their 



