108 
GEOLOGY OF THE FOURTH DISTRICT. 
The Orthis striatula of the Trenton limestone differs from this shell in the less convexity 
of the lower valve, and in the more strongly marked radii which are crenulated, or crossed by 
elevated lamellae which extend across the depressions between them ; the beak is less incurved 
and the shell more circular in form. 
Localities —Niagara, Lockport, Sweden, Rochester, Wolcott. 
7. a. b. c. Ortliis hybrida. Silurian Researches , pi. 13. f. 11. Lenticular, radiated; radii 
dividing towards the margin, lower valve most convex ; upper one slightly depressed in front; 
front rather straight or a little impressed ; area narrow; hinge line short. 
The specimen figured is somewhat larger than the one in the Silurian Researches, but the 
more abundant form is like that. It is usually convex near the beaks and flattened toward 
the margin, the valves appearing equal. The shell occurs with O. canalis and the other forms 
here figured, few of them rising above the shale of the Niagara group, and none of them 
known above the Niagara limestone. 
Locality —Lockport, Sweden, Rochester and Wolcott. 
Atnjpa affinis. (Terebratula affmis, M. C. t. 324, f. 2. 
A. reticularis, Dalman, Hisinger, &c.) 
The illustration presents the common form of this va¬ 
riable species as it occurs in the shale of the Niagara 
group. The shell is marked by about twenty-four radii 
which bifurcate about one-third of the distance from the 
beak to the margin. The whole surface is closely imbricated by the projecting laminae of the 
shell. This structure is visible in the enlarged portion. 
This shell is common in all localities and abundant in many places. It is usually much 
flattened, rarely presenting the rotund form of the figure. Several other species of this genus 
are common in this group, and some of them will be figured at the end of the volume. 
1. Orbicula ? squamiformis, n. s. — Oval, depressed ; apex nearer to one extremity of the 
shell ; surface covered with concentric lines, which diverge on the posterior part of the shell. 
Shell translucent. Longest diameter of an inch. 
