INVERTEBRATE PALAEONTOLOGY. (Jl 



2. breviarca, Conrad. 



Shell small, suborbicular or suboval, more or less rounded at the 

 extremities, or rarely truncated obliquely behind; surface nearly 

 smooth, or finely radiately-striated ; hinge-area with minute cross- 

 striae; hinge-denticles very fine and crowded. — T. perovalis, Conrad. 



It is possible that the little shells composing the Breviarca group should 

 be separated genetically from the typical trigonarcs. Their smaller size, 

 generally shorter form, smoother or minutely striate surface, finely cross- 

 striated area, and particularly their fine, crowded hinge-denticles, give them 

 a peculiar aspect, contrasting rather decidedly with the larger typical group. 



The genus Trigonarca is pre-eminently characteristic of the Cretaceous 

 system, in which numerous species are found. It seems, however, to have 

 attained its greatest development, both as to the number of species and the 

 large size of some of them, in the Cretaceous rocks of Southern India, where 

 only the typical section appears to occur, unless some of the species referred 

 by Dr. Stoliczka to Axincea may belong to the Breviarca group. Both sections 

 occur in the Cretaceous of this country. 



So far as yet known, no species of this genus occurs in either older or 

 more recent formations than the Cretaceous. It seems to hold an interme- 

 diate position between Cucullcea, Axincea, and Noetia; most nearly resembling 

 the latter, to which the typical section is very closely allied. It differs from 

 Noetia raainly in its arched instead of angular hinge-line, straighter hinge- 

 denticles, smoother surface, and in having a slightly-projecting edge to its 

 posterior muscular scar. 



The little shells included in the Breviarca group most nearly approach 

 Axincea, but have generally a less regularly-rounded outline, more gibbous 

 valves, finer and more crowded hinge-denticles, and also differ in having the 

 hinge-area minutely cross-striated, and the edge of the posterior muscular scar 

 a little more projecting, somewhat as in Cucullcea* It would be difficult, 

 however, to distinguish some of the round species from Axincea without 

 examining the hinge-area and interior, while others more nearly approach the 

 typical forms of the genus. 



* In some species, there are faint indications of a similar projection of the posterior margin of the 

 anterior muscular scar also. This is the case with Mr. Conrad's /•'. mnbunata as well as our II. exigua. 



