360 PALEONTOLOGY 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 Leiorhynchus quadi-icostata. 

 Smaller specimens fi-om Widdcr 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. mesacostaUs of the Chemung group. 



Geological fotTiialion 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 Canaudaigua 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. 



Leiorliynclins iris (n. s.). 



PLATE LVi; 



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 striae and a few imbricating lines of 

 growth. 



Oeolor/ical formation and locality. In the calcareous shales of the age of the 

 Hamilton or Chemung formations, near Rockford, Indiana. Eeceived from Mr. 

 O. St. John. 



