412 THE PALEONTOLOGY OF MINNESOTA. 
[Leptsena unicostata. 
other valve. Interior with the hinge teeth not prominent; rostral cavity with a 
pair of small depressions, in front of which are two elongate and well marked scars 
of the adductor muscles; surrounding these, with their lateral margins strongly- 
elevated, are the large bilobed and striated diductor scars, which continue forward 
for two-thirds the length of the valve; surface outside the muscular area covered 
with strongly elevated, oblique and minutely perforated pustules, more or less 
radially arranged; space underneath the cardinal area, on each side of the teeth, 
filled up with shell matter which, towards the extremities, is reflexed and becomes 
obsolete on the lateral portions of the valve. 
"Dorsal valve with the disc or visceral region flattened and, like that of the 
other valve, without any traces of concentric undulations; deflected anterior and 
lateral margins conforming nearly to those of the other valve; beak nearly obsolete; 
area linear and provided with a marginal furrow for the reception of the edge of 
the other valve; cardinal process rather small, cordate or bilobed, with the [crenated] 
socket on each side for the reception of the teeth of the other valve well defined; 
interior with [a pair of medially divided adductor] muscular scars generally moder- 
ately distinct and separated by a small mesial ridge [which is nearly obsolete a little 
posterior to the mid-length, with a small, shallow scar on each side, the septum then 
again becomig prominent and continuing to the geniculated margin]; other parts of 
the visceral region occupied by rather crowded [oblique and rather large] granules. 
Surface of both valves ornamented by fine, crowded, radiating striae, which increase 
by intercalation and division, while one of those on the middle of the ventral 
valve is generally five or six times as large as the others, and really forms a dis- 
tinct rib." 
Adult Minnesota specimens referred to this species have concentric corrugations 
on the central flat disc, the latter being more convex than in Illinois examples. 
Associated with the large shells are also numerous smaller ones, which are compar- 
atively narrower, more mucronate and without wrinkles. Since these specimens are 
immature, and certainly of the same species as the larger ones, there is no hesitation 
in extending the specific description of L. unicostata so as to contain the corrugated 
examples. From L. rhomboidalis, var. tenuistriata Sowerby, this species can be dis- 
tinguished only by the obsolete or inconspicuous wrinkling, the large mid-rib and, 
when the interior is shown, by the very large, bilobed, diductor scars of the ventral 
valve. 
In the upper portion of the Hudson River group of the Ohio valley L. rhomboi- 
dalis, var. tenuistriata is a very common form, but i^ replaced by L. unicostata in 
the same formation in northern Illinois, Wisconsin, Iowa and Minnesota. The 
latter species, it seems, must be regarded as a branch of the line leading to L. 
