412 THE PALEONTOLOGY OF MINNESOTA. 



[Li-ptii'na 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 is 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. 



