CRETACEOUS FOSSILS. 403 



fibrous coating and an inner nacreous or pearly lining; hinge line straight, 

 edentulous, with a narrow linear external cartilage area on each valve. 

 Valves provided with an oblique internal rib passing from behind the 

 beaks, along the postero-cardinal slope, toward the postero-basal margin, 

 marking the position of the posterior muscular imprint; other muscular 

 markings unknown. Type, E. typica Whitf. 



We have not been able to detect the anterior muscular scar or pallia! 

 line, except the latter as an irregular transverse line of small tubercles 

 across the beak of internal casts in several cases, and a slight curving 

 ridge, faintly marked on a smaller number of examples, passing from the 

 apex along the anterior side for a short distance. The genus differs from 

 Inoceramus so far as yet known only in the existence of the internal rib. 

 The species now known are E. alveatus rz Inoceramus alveatus Morton ; E. 

 impressus z= I. impressus D'Orb. ; E. sidcatus — I. cripsii var. sulcatus Rcemer, 

 and E. typica Whitf, herein described. 



EKDOCOSTEA TYPICA. 



Plate 9, figs. 1-7. 

 Endocostea typica Whitf., Prelim. Eept. Pal. Black Hills, 1877, p. 32. 



Shell of medium size, transversely-subovate, oblique and slightly in- 

 equivalve, the right side being the largest in the only example where they 

 are preserved in contact. Valves strongly ventricose at the anterior end 

 and on the anterior part of the umbonal region, becoming more compressed 

 and attenuate behind and along the posterior cardinal portion, sometimes 

 strongly depressed and concave between the cardinal line and dorsal slope. 

 Hinge line straight, often two-thirds as long as the shell below and slightly 

 rounded at the extremity; beaks small, incurved, projecting but little above 

 the cardinal line and situated near the anterior end, but not quite terminal; 

 anterior end short, sharply rounded at the antero-cardinal angle, and 

 extending a little forward below for about half the height of the valve, 

 when it rounds backward to the basal line, the upper part forming, with 

 the cardinal line, an angle of about 95° to 100° ; posterior end nearly 

 equal to or narrower than the anterior, and more sharply rounded, most 

 strongly from below; basal line forming nearly half of an ellipse or semi- 



