32 



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 marking unknown. Type, = E. Typica, Whitf. 



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

 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 = Inoceramus alve- 

 atus, Morton ; E. impressus = I. impressus, D'Orb. ; E. sulcatus = I. cripsii 

 var. Sulcatus, Eoemer ; and E. typica, Whitf., herein described. 



Endocostea typica, n. sp. 



Plate 9, figs. 1-7. 



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

 quivalve, the right side being the largest in the only example where 

 they are preserved in contact. Valves strongly ventricose at the ante, 

 rior end and on the anterior part of the umbonal region, becoming more 

 compressed and attenuate behind and along the piosterior cardinal por- 

 tion, 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 a,bove 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 semiovate, the widest 

 part varying from in front of to behind the middle of the length, in 

 different individuals. Surface of 'the valves usually marked by very 

 distinct concentric undulations, though sometimes they are only moder- 

 ately developed. Oblique internal rib strongly marked, narrow and 

 pointed near the beak, and gradually widening and deepening posteri- 

 orly, the concentric furrows distinctly crossing and leaving, on the 

 deeply concave channel of the casts, corresponding depressions. Inner 

 layers of the shell highly nacreous ; outer prismatic layer very thin. 



This species resembles Inoceramus Barabini, as it is recognized in the 

 same region, in the general form and in the subcimeate anterior end of 

 the shell; but the concentric furrows are generally more distinctly 

 marked and regular. It is readily distinguished by the oblique sulcus 

 and by the opposite valve being the largest, if the example above refer- 

 red to, and figured on the plate, is rightly interpreted. 



