S56 PALiEONTOLOGY OF NEW- YORK. 



Genesee slate, and occurring generally in the more shaly portions of the 

 Chemung group. 



Leiorli}iiclias liniitaris. 



PLATE LVI. 



Orthit limitaru : Vahcxem, Geol. Report Third District Now- York, p. 146, f. 3. 1842. 

 Mrypa Hmitaru : Hall, Geol. Report Fourth District New-Tork, p.l82, f. 11. 1843. 

 Ltiorhynchut limitarU : Hall, Thirteenth Report on the State Cabinet, p. 85. 1860. 



Shell ovate, suborbicular or transverse, moderately or extremely gib- 

 bous, with the sinus and mesial fold more or less developed. 

 Ventral valve in young shells scarcely less convex than the opposite, 

 becoming gradually more dissimilar with the growth. In old shells 

 the ventral valve is rather gibbous near the beak, and gradually 

 depressed below, becoming deeply sinuate, produced in front and 

 abruptly truncate ; beak closely incurved. 

 Dorsal valve more convex than the opposite, gibbous in older shells, 

 the mesial fold becoming developed below the middle of its length ; 

 umbo gibbous, and often rising nearly as high as the beak of the ven- 

 tral valve. 

 Surface marked by numerous angular or subangular plications, those of 

 the mesial fold and sinus distinctly bifurcating, while sometimes a few 

 of those on the sides are divided ; concentrically marked by fine striae 

 as in all the species of this genus. 



In the youngest specimens observed, the ventral valve below the 

 umbo is flat or concave ; while in those a little older, the two valves are 

 nearly equal, and the shell is nearly circular. The illustrations on Plate 

 LVI present the principal varieties of form of this species. 



Geological formation and localities. This species is abundant in the Goniatite 

 limestone of the Marcellus shale ; and in the absence of this limestone, it is 

 abundant in certain layers of the Marcellus shale, always characterizing that hori- 

 zon, a fact which induced Mr. Vanuxem to give it the name limitaris. It is 

 widely distributed, occurring in the Gtoniatite limestone in Schoharie, and in the 

 same position at intermediate points as far as Marcelhis in Onondaga county. It 

 is abundant in the shale at Avon, at Lcroy, near Alden, and at numerous other 

 places within the State of New- York. I have not seen specimens from any western 

 locality. 



