TRENTON LIMESTONE. 



157 



Genus MODIOLOPSIS. 



[Modiola, and o^]/l^, appearance; from its similarity to Modiola,] 



Character. Equivalve, inequilateral, elongatetl, becoming broader posteriorly ; uniboncs 

 near the anterior extremity, which is marked by a single strong muscular impression as in 

 Modiola. A sinus often extends from the anterior side of the umbones, obliquely back- 

 wards, leaving the anterior portion separated as a kind of lol)e. Surface marked by fine 

 concentric stri;e ; shell thin. 



Tliis genus, as defined, includes a very natural group of shells found in the older silurian 

 strata, some of which have been referred to Cvpricardia, Modiola, Pterinea, and other 

 genera. One of the most prominent characters is the strong muscular impression, which is 

 close to the anterior margin : this is often visible in the shell, forming a little circumscribed 

 elevation, and more conspicuous in the cast, where it is usually well preserved. There is 

 often a slight contraction or sinus below, or posterior to, the umbones, Itut tliis is not always 

 conspicuous. The shells of this genus are, for the most part, smooth, or marked only by 

 fine concentric lines, indicating the lamiiue of the shell, and they are generally free from 

 angular ridges. Cypricardites modiolaris (Conrad).* 



200. 1. MODIOLOPSIS MYTILOIDES ( 71. . ?;?.). 



Pl. XXXV. Figs. 4 a, b. 



Subcyhndrical, enlarging posteriorly ; cardinal line extending about half way from the 

 umbones to the posterior extremity ; umbones small, compressed anteriorly, with a slight 

 depression extending obliquely backwards, and producing a small sinus in the ventral 

 margin ; anterior extremity rounded, with the muscular im]iression reaching to the margin 

 (in the cast) ; posterior extremity becoming broader, and sloping rapidly from the extremity 

 of the cardinal line to the ventral margin ; surface marked by fine concentric lines; cast 

 smooth. 



This shell is distinguished by its elongated cylindrical furm, and gradual enlargement 

 towards the jiosterior extremity. The anterior side is small, and somewhat abruptly nar- 

 rowed. It is usually found as casts, presenting some remains of the concentric markings ; 

 the musculnr impression is nearly oval, acute above, and contiguous to the margin of the 



shell. 



Fig. 4 a. Right valve of a specimen, showing the muscular impression. 

 Fig. 4 b. Dorsal view of the same specimen. 



Position and locality. In the thin slialy beds near llie upper part of the Trenton limestone, 



Middleville. (Cabinet of Mr. V.4.su.\em.) 



* I find myself compelled to abandon the use of the name Cypricardites, as applied to shells difiering so widely as 

 these do from the Cypricardia, and belonging apparently to the Monomvaria and not Dimyaria. So far as it is 

 possible to ascertain, none of the species of the older strata possess two muscular impressions, and therefore do not 

 strictly fall under the genu.s Cypricardites of Conrad (^nn. O'col. Report, isll, p 51). 



