Foerste.] 
314 
[May 1, 
Kentucky. Specimens which cannot be distinguished from it spe- 
cifically occur at Collinsville, Alabama. Interior and exterior 
casts of the ventral valve have been found. The mesial sinus is 
moderate ; bordered on each side by strong radiating plications, 
rounded on top. In addition to these two plications two more occur, 
one on each side of the shell, not originating at the apex of the 
beak, but at some distance below the same ; these plications are 
well defined on the side towards the sinus, but not very distinctly 
on the opposite side. Faint indications of a third plication exist, 
and sometimes an obsolete fourth one at the postero-lateral mar- 
gin. The surface of the valve is marked by numerous, fine, radi- 
ating striae. 
The length of the largest ventral valve is 16.5 mm. ; the width 
15.5 mm. ; the width of the sinus at the anterior margin of the shell, 
5 mm. ; the elevation of the beak above the postero-lateral angles 
about 4.5 mm. 
Atrypa reticularis, Linnaeus. 
Atrypa reticularis is described by Hall from the Clinton Group of 
New York. Atrypa impressa , Shaler, from Anticosti, is a form of 
the same size as the figured New York specimens or even twice their 
diameter. It belongs to the medium striated forms, about two and 
one-half striae occupying a width of 2 mm. 
At Cumberland Gap, Tennessee, this species is found, with an 
average length of 13.3 mm. and a breadth of 15 mm. The ventral 
valve is of medium convexity, with twenty-four to thirty -six radi- 
ating striae, crossed by concentric striae, forming slight thickenings 
of the plications as they pass over them, giving rise to the concen- 
tric rows of low elevations so characteristic of this species. The 
dorsal valves are not represented in our collections. 
Atrypa marginalis, Dal man. 
(PLATE VI, FIGS. 8, 9.) 
Atrypa marginalis , Dalman, was described from Sweden, but is 
also plentiful in Great Britain. 
At Brown’s Quarry, near New Carlisle, Ohio, substantially the 
same species is found, having the following characteristics. Shell 
of about the same length and breadth or the breadth slightly ex- 
ceeding the length. From the beak to the postero-lateral angles, 
the posterior margin of the ventral valve is slightly incurved, the 
