FOSSILS OF THE HUDSON KIVEE GKOUP. 85 



MODIOLOPSIS PHOLADIFORMIS. 



Plate 2, flg. 16. 



Modiohpsis pholadiformis, Hall; Fost. and Whit., Lake Sup., p. 213, pi. 30, fig. 1, and 

 pi. 31, fig. 1. 



Shell of medium size or larger, sub-ovate in outline, widest posteriorly ; 

 the length usually about twice as great as the breadth. Valves depressed 

 convex, but usually flattened by compression ; umbonal ridge slight, 

 sub-angular in the upper part, becoming almost obsolete behind ; anterior 

 portion having a slight sulcus crossing the valves from the beak, and 

 reaching the base at or within the anterior third of the length. Hinge 

 line arcuate ; anterioj end contracted below the beaks, slightly pro- 

 longed, and narrowly rounded in the lower part ; basal line contracted at 

 about a third of its length from the anterior end, corresponding with the 

 sulcus crossing the shell, and strongly curved upwards toward the poste- 

 rior extremity ; posterior margin obliquely truncate ; longest at the pos- 

 tero-basal angle, and rounded toward the extremity of the hinge line. 

 Beaks small, compressed, slightly angular, and projecting but little 

 above the hinge. 



Surface of the valves marked with numerous irregular, concentric lines 

 of growth, strongest on the anterior end ; and also by low, rounded, some- 

 what irregular, and occasionally bifurcating plications, which diverge 

 from the umbonal ridge, curving gently backwards in their course toward 

 the basal line, and more strongly to the cardinal margin. Anterior to 

 the sulcus of the valves these diverging plications have not been noticed, 

 and it is probable that they do not occur on this part of the shell. 



The species has about the same general form and outline as that of 

 M. modiolaris, but differs very materially in the existence of these diverg- 

 ing plications of the surface, and also from any other shell found asso- 

 ciated with it. 



Formation and locality : In the soft shales of the upper part of the Hudson River 

 group, nearWaynesville, Ohio. We have not seen specimens of this species from any 

 other locality in Ohio, but it is probable that they may occur in most localities of the 

 upper beds of the formation, as it appears to have a somewhat extended geographical 

 range ; the original specimens of the species being from the Little Bay de Noquet, at 

 the upper end of Green Bay, Lake Michigan. 



