261 

 Genus SPHENOTUS. Hall. 



[Ety. : Sphen, wedge; oiis, ear.] 

 (1884: Pal. N. Y., Vol. V., Pt. I., p. xxxiii.) 



Shells with equal, very inequilateral, valves, elongate sub- 

 trapezoidal or cylindrical in outline. The anterior end is 

 short, and the posterior end usually olilifjuely truncate. A 

 long", straight hinge line; a well-marked umbonal ridge, 

 with a secondary ridge usually occurring on the slope above 

 it; a more or less defined oblique cincture; two short cardi- 

 nal, and two slender lateral, teeth ; external ligament, lodged 

 in a slender groove, and concentric surface striae, character- 

 ize this genus. 



Sphenotus truncatus. (Conrad.) (Fig. 180.) (Pal. 

 N.y., Vol. v., Pt. I., p. 394, PL LXY.) 



Distinguishing Cliaracters. — Small size; straight base, 

 slightly constricted anterior to the middle; augular umbonal 



Fig. 180. Sphenotus truncahis. Specimen retaining both valves, x 2 : left and right 

 valves (from Hall). 



ridge; obscure umbonal fold; shallow cincture; very fine 

 radiating striae. 

 Found in the Demissa bed, at Section 5 (very rare). 



Genus CONOCARDIUM. Bronn. 



[Ety.: Konos, cone; kardia, heart.] 

 (1835: Lethaea Geognostica, Vol. I., p. 92.) 



Shells with equal inequilateral valves of a trigonal or 

 spindle-shaped outline. The anterior* end is obliquely trun- 

 cated, and along the cardinal line it is produced into a tube, 

 which is often broken away, leaving an opening. The 



* I follow Zittel in regarding the truncated end as the anterior one. 



