31 



gether in the same localities and presenting only slight differences, as 

 one and the same species, than to identify these far western forms with 

 species described from Europe, as has been done by so many authors. 

 We do not dispute their great resemblances, but believe them to be 

 rather representative species than equivalent forms. 



INOCERAMUS PERPLEXUS, n. sp. 



Plate 8, fig. 3, and plate 10, figs. 4 and 5. 



Shell rather below the medium size, erect, truncate, subovate in out- 

 line, shortest at the hinge-line, and gradually expanding to below the 

 middle of the height ; valves convex ; beaks terminal, slightly inclined 

 forward, pointed, and incurved ; hinge-line short and straight, not 

 exceeding two-thirds of the length below in any of the examples seen ; 

 anterior border straight or very slightly concave for more than half the 

 height of the shell, where it unites with the deeply rounded basal 

 border; posterior margin of the shell gradually widening from the 

 extremity of the hinge-line to the point of greatest width, in an antero- 

 posterior direction, which is rather below the middle of the height, 

 where it unites in a regular curve with the basal margin. Valves most 

 strongly convex along the umbonal ridge, which is situated nearest to 

 the anterior side of the shell and slopes gradually to the postero- 

 cardinal portion, where it becomes almost flattened ; while the anterior 

 border is vertical, or sometimes slightly concave, for more than half the 

 depth of the valve. 



{Surface of the shell marked by strong, irregular, concentric folds or 

 undulations, parallel to the margin and passing off on the vertical 

 anterior border. Substance of the inner layers highly nacreous. Fibrous 

 coating, when preserved, apparently thin. 



This species is somewhat remarkable for its erect form and strong 

 concentric undulations. In form it resembles the specimens of I. fragilis 

 Hall and Meek, given by the latter author, (Pal. U. S. Geol. Surv. 

 Territ,, p. 42, figs. 1 2), but in the surface characters it is entirely dis- 

 tinct from that species, and, when compared with examples of that 

 species having the characters of the original specimen, is very readily 

 distinguished. It also somewhat resembles I. JSllioti Gabb from the 

 Cretaceous sandstones of San Francisco Harbor, Gal. 



Formation and locality. In concretionary limestone at the top of 

 shales of Cretaceous Xo. 2, on the Belle Fourche, about ten miles west 

 of Crow Peak, Black Hills, Dakota. 



ENDOCOSTEA, new gen. 



Shell resembling Inoceramus, being bivalve, with both sides convex, 

 but more or less unequal, and composed of two layers, an outer verti- 

 cally fibrous coating and an inner nacerous or pearly lining ; hinge-line 

 straight, edentalous, with a narrow linear external cartilage area on 



