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PALEONTOLOGY OF NEW YORK. 
with the muscular area not strongly limited ; consisting of two broad flabel- 
late diductor scars enclosing an elongate, more distinctly defined adductor. 
The faintness of the limitation of this area is in marked contrast to the sharply 
defined muscular area in the corresponding valve of Lept^jna. In the brachial 
valve the cardinal process is more closely sessile than in Lept^na, and there is 
frequently a linear callosity between the branches. The posterior adductor 
scars have the arborescent markings of Leptoena rhomboidalis, and these impres¬ 
sions are the only ones well defined, the anterior scars being narrow and rarely 
retained with distinctness. From the anterior margin of the muscular area 
radiates a series of irregular furrows and nodose ridges, which are to some ex¬ 
tent of vascular origin. 
Type, Leptcena alternatei, Conrad. Trenton and Hudson River groups. 
After Davidson. 
Observations. There are some shells, a small number of species however, 
which combine to some extent the characters of both Lept^na and Rafines- 
quina. We may instance Leptma deltoidea, Conrad, and Strophomena unicostata, 
Meek and Worthen, in which there are not only low, concentric corrugations 
on the exterior, but in the latter species the interior of the brachial valve 
has more distinctly the impress of Lept^ina than of Rafinesquina. There 
are concentrically wrinkled species in the genera Strophomena, Stropheodonta 
and Strophonella, but that character will prove of little value except for a 
subsidiary grouping of species. The extravagant development of this feature 
