LAMELLIBRANCHIATA. 275 



Valves convex, not gibbous ; greatest convexity about the middle of the 

 shell. 



Hinge-line oblique, extending for more than half the length of the shell. 



Beaks sub-anterior, not prominent, directed forward. Umbonal region 

 convex, sometimes approaching to gibbous, not strongly defined. 



Test thick, marked by concentric striae. 



Anterior muscular impression strong. Pallial line, in the cast, marked by 

 a ridge extending parallel to the basal margin. 



A medium-sized specimen has a length of 60 mm., and a height of 35 mm. 



This form is proportionally longer than M. Schoharie, and broader than M. 

 cymhula, which is also sub-nasute on the posterior end, with more depressed 

 beaks. 



Formation and localities. In the limestone of the Upper Helderberg group, 

 near Columbus, 0., and Cayuga, Ont. 



MODIOMOKPHA CONCENTRICA. 



PLATE XXXIV, FIGS. 9,10; PLATE XXXV, FIGS. 1^; and PLATE XXXVI, FIGS. 1-16 (17, 18?). 



PteHneaeoneentrica, Conrad. Geol. Surv. N. Y., Ann. Rep., p. 116. 1838. 

 Oyprieardites eoneeiUrica, Cosbad. Geol. Surv. N. Y., Ann. Rep., p. 52. 1841. 



" oblonga, " Geol. Surv. N. Y., Ann. Rep., p. 52. 1841. 



Modiola concentrica, " Hall. Geol. Sui-v. N. Y., Rep. Fourth Dist., p. 196, tab. 78, fig. 9. 1843. 



Modioinorpha concentrica, " " Prelim. Notice Lamellibranchiata, 2, p. 73. 1870. 



" " " " Pal. N. Y., vol. V, pt. 1. Plates and Explanations: PI. 34, figs. 



9, 10 ; pi. 36, figs. 1-16 (17, 18 ?). 1883. 



Shell of medium size, ovate, extremely variable in its proportions ; length less 

 than twice the height ; basal margin often nearly straight, usually a little 

 concave on the anterior third ; posterior margin abruptly rounded below 

 and more gently curving above ; cardinal margin oblique in the prevailing 

 forms, moderately arcuate, often nearly straight, sub-alate in many specimens. 

 Anterior end produced beyond the beaks, abruptly rounded, sometimes 

 nasute, limited by a broad depression extending from the beak to about the 

 anterior third of the basal margin. 



Valves moderately convex, gibbous along the umbonal slope ; the point of 

 greatest convexity is about the anterior third of the length of the shell. 



