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PALAEONTOLOGY OF NEW-YORK. 
mesial fold, while those on the sides of the shell are obscure or nearly obsolete ; 
and were they imbedded in an argillaceous shale instead of a calcareous one, they 
would not be very different in appearance from Leiorliynchus qucidricostatci. 
Smaller specimens from Widder present almost- every phase of development in 
the plications. 
Altogether, accepting for the present the distinction between these forms, I 
believe they will prove identical; and I am not indisposed to unite with these 
the L. mesacostalis of the Chemung group. 
Geological formation and localities. This species occurs in the Hamilton group, 
from Schoharie county to the western limits of the State. It is common at Fulton- 
ham and Summit; at Plainfield and Unadilla forks ; and also on the shores of 
Owasco, Cayuga, Seneca and Canandaigua lakes. It is less common at York, Mos¬ 
cow, Darien and Eighteen-mile creek. The specimens from Canada West are less 
compressed than those of New-York. 
Leiorliynchus iris (n. s ). 
PLATE LYI. 
Shell small, subangularly ovoid, with a deep sinus and prominent mesial 
fold. 
Ventral valve convex on the umbo, flattened on the sides and deeply- 
depressed in a median sinus below the middle ; apex elevated, perfo¬ 
rate, the lower side of the foramen bordered by deltidial plates with a 
distinct narrow area on each side. 
Dorsal valve gibbous in the middle, curving abruptly to the sides, and 
strongly elevated in a mesial fold towards the front. 
Surface plicated, the plications subangular, beginning at one-third the 
length from the apex, some of them obscurely bifurcating; concen¬ 
trically marked by fine elevated strioe and a few imbricating lines of 
growth. 
Geological formation and locality. In the calcareous shales of the age of the 
Hamilton or Chemung formations, near Rockford, Indiana. Received from Mr. 
O. St. John. 
