TRENTON LIMESTONE. 
157 
Genus MODIOLOPSIS. 
[ Modiola, and appearance; from its similarity to Modiola.] 
Character. Equivalve, inequilateral, elongated, becoming broader posteriorly ; umbones 
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 lobe. Surface marked by fine 
concentric strife ; shell thin. 
This genus, as defined, includes a very natural group of shells found in the older silurian 
strata, some of which have been referred to Cypricardia, 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, but this is not always 
conspicuous. The shells of this genus are, for the most part, smooth, or marked only by 
fine concentric lines, indicating the laminae of the shell, and they are generally free from 
angular ridges. Cypricardit.es modiolaris (Conrad).* 
200. 1. MODIOLOPSIS MYTILOIDES (n. sp.). 
Pl. XXXV. Figs. 4 a, b. 
Subcylindrical, 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 impression 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 form, and gradual enlargement 
towards the posterior 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 muscular 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 shaly beds near the upper part of the Trenton limestone, 
Mlddleville. • ( Cabinet of Mr. Vanuxem.) 
I find myself compelled to abandon the use of the name Cypricarditcs, as applied to shells differing so widely as 
these do from the Cypricardia, and belonging apparently to the Monomyaria 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 genus Cypricardites of Conrad {Ann. Geol. Report, 1S41, p. 51). 
