INVERTEBRATE PALAEONTOLOGY. 95 



between those that are costate and those that are not, that it would be diffi- 

 cult in separate them into subgenera on this character. 



This irenus is closely related to Trigonarca of Conrad, particularly to the 

 subgenus Breviarca From the first it is distinguished by its more rounded, 

 compressed form and regularly-convex valves, without prominent posterior 

 umbonal slopes. From the Breviarca group it also differs, though less strongly, 

 in the same characters of form, and in having much coarser and less crowded 

 hinge-denticles, and in having its cardinal area divaricately furrowed instead 

 of merely marked by very fine cross-strife. The breviarcs are also all smaller 

 shells than the average size of Axincea. 



The genus Axincea'is said to occur in the Trias and Jura, but the species 

 from these rocks are few and not very well known. It is well defined, how- 

 ever, and not uncommon in the Cretaceous. It also ranges through the 

 Tertiary, and probably attains its greatest development in the existing seas. 

 The living species are most abundant on the coast of America, but they like- 

 wise occur in European seas as well as on the coasts of Africa, Australia, &c. 



I \ i 1 1 be i > siibimbricata, M. &H. 



Plate 23, figs. 14, a, 1>, e. 



Petunatlus subimbricatus, Meek and Hayden (18571, Proceed. Acad. Nat. Sci. Pbilad., IX, 14G. 

 Jxinan subimbriaila, Meek and Hayden (1860), ib., XII, 185. 



Shell subcircular, or slightly longer than high, moderately convex; extrem- 

 ities rounded, the posterior side being sometimes a little broader and less regu- 

 larly rounded than the other; base forming a very nearly semicircular curve, 

 and strongly crenulate within ; hinge-area rather small, marked by about 6ve 

 to seven distinct grooves ; beaks almost central, rather gibbous, obtuse, and 

 neither oblique nor distinctly incurved ; hinge a little arched, teeth distinct, 

 lateral ones oblique and curved, the concave sides being turned outward from 

 the middle; muscular impressions rather large, but shallow. Surface orna- 

 mented by distinct lines of growth, crossed by numerous regular, simple, 

 depressed, radiating costaa, about twice as broad as the grooves between. 



Length, 1.55 inches; height, 1.36 inches; convexity of the two valves, 

 1.03 inches. 



On the surface of specimens a little weathered, the concentric markings, 

 which are slightly arched in crossing the costae, seem to be rather imbricating 

 near the borders of the shell; this character, however, may not be conspicu- 



