194 



Distinguishing Clmracters. — Hin<2,e line shorter than the 

 greatest width of the shell; extremities roniided ; length and 

 width about as two to three ; surface striae distant. 



Found in the Stictopora and Deniissa beds, at Section 5 

 (rare); in the shale below the Athyris spiriferoides bed, at 

 Section 7 (rare): and in the Pleurodictyuni beds, in Avery's 

 Ravine (rare). 



Genus STROPHEODONTA. Hall. 



[Ety. : Strop/iei'x, hinge ; odous, tooth.] 

 (1852: Pal. X. Y., Vol. II., p. G:i 1892: Pal. X. Y., Vol. VIII., Pt. I., p. 284.) 



Shell concavo-convex, the convex valve l)eing the pedicle 

 valve. Outline varying from semi-circular to semi-elliptical, 

 with the hinge line us nally equal to or greater than the greatest 

 width of the shell. Area of the pedicle valve higher than that 

 of the brachial valve, both margins furnished with project- 

 ing denticulations, which interlock and form articulations. 

 Muscular areas well marked and variously bounded. A 

 strongly marked, commonly bifid, cardinal process occurs in 

 the brachial valve. 



A number of sub-genera are recognized. 



Stkopheodoxta DEMissA. Tourad. (Fig. 98.) (Pal. X.Y., 

 Vol. lV.,p. 101. PI. XVII.) 



Distinguishing Clnirnrters. — Medium size: pedicle valve 

 stnmoiv convex; concavity of the brachial valve less; leng-th 

 and width nearly equal; surface striiP numerous, stronger 

 and more elevated near the beak, increasing by intercalation 

 and bifurcation towards the front: muscular impressions of 

 the pedicle valve consist of large flabellate divaricators, 

 separated towards the front and distinctly lobed. and small 

 adductors lying between them separated from each other by 

 a depression ; in the brachial valve the adductor impressions 

 are divided longitudinally by a narrow ridge, and often 

 limited in front by elevated ridges. 



Found in the Encrinal limestone, at Section 5 (one speci- 

 men Ji; the Stictopora bed, at Sections 5 and 6 (rare); the 



