LAMELLIBRANCHIATA. 267 



M D I L A , Lamarck, 

 Sub-Genus MYTILOPS, Hall. 



MODIOLA (MYTILOPS) PR/ECEDENS. 



PLATE XXXIII, FIGS. 9-1S 



Modiola prOBcedens, Hall. Prelim. Notice of the Lam. Shells, etc., pt. 2, p. 1. 1869. 



MytUnps (Modiola) prWeedens, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations : PI. 33, tigs. 9-18. 



Jan., 1883. 



Shell of medium size ; body elongate, semi-elleptical, broadly and obtusely 

 sub-triangular ; length a little more than twice the height ; byssal margin 

 slightly sinuate ; anterior end compressed, abruptly rounded, nearly straight 

 or gently curving along the ventral side, acutely recurved at the posterior 

 extremity, thence broadly rounded to the extremity of the hinge. 



Valves equal, convex below, becoming gibbous and angular anteriorly. 

 The line of greatest convexity extends from the beak to the post-ventral 

 extremity, slightly arcuate. 



Hinge-line slightly arcuate, sub-alate ; length nearly half the length of 

 the valve. Beaks small, sub-terminal, slightly incurved. 



Test marked by fine, equal, concentric strise, and by fine, wrinkled radii, 

 which diverge along the median line and curve toward the opposite margins. 



Ligamental area with one or two longitudinal grooves. 



Interior unknown. 



A small specimen has a length of 28 mm., and height 12 mm. A medium- 

 sized specimen has a length of 36 mm., and height 17 nun. 



Formation and localities. In a conglomerate of the Chemung group, Port- 

 ville, Alleghany county, N. Y., and south-east of Smethport, Pa. 



