HAMILTON GROUP. 
199 
4. Atrypa prisca. (Terebratula affinis, Min. Con.; T. prisca, Yon Bucii ; T. reticularis, 
Bronn, Lethea Geog.; Atrypa reticularis, Dalman ; A. affmis, Silurian Researches .) — 
Oblong, often nearly circular ; lower valve least convex, with the beak scarcely prominent, 
and pressed close to the beak of the upper valve; upper valve very convex; front margin 
often advanced and a little depressed; surface radiated with numerous round striae, which 
bifurcate at irregular intervals. 
The specimens vary in size, and very frequently are flattened from compression, so that 
they do not present the rotund form of the figure. This is an abundant and widely distri¬ 
buted fossil, appearing throughout the Hamilton and Chemung groups. It is more abundant 
in the Hamilton group just below the Encrinal limestone, associated with the preceding and 
succeeding forms. It also occurs in the Moscow shale in great numbers. 
Localities —Seneca lake ; Hopeton ; Moscow ; York ; Darien ; Eighteen-mile creek, &c. 
5. Atrypa concentrica (Bronn, Lethea Geog.). — Spheroidal, flattened or rotund, impress¬ 
ed in front; upper valve very convex in the middle, slightly elevated in front and depressed 
at the sides ; beak scarcely prominent, and closely pressed to the beak of the lower valve ; 
lower valve convex, with a prominent, slightly incurved beak, and a mesial sinus extending 
two-thirds the length of the shell, elevating the front; surface marked by numerous concentric 
stride, which in young specimens are projecting lamellae, giving the shell a squamose ap¬ 
pearance. 
This is an exceedingly abundant fossil in many places, and appears in the greatest num¬ 
bers just below the Encrinal limestone. Sometimes it occurs in the shale above this stratum, 
associated with Orthis umbonata. It will be readily recognized from its general smoothness 
and numerous concentric lines. 
Localities —Seneca and Cayuga lakes ; Hopewell; Conesus outlet; Darien; and in greater 
numbers and perfection at Eighteen-mile creek on Lake Erie. 
