S84 PALEONTOLOGY OF NEW-YORK. 



width a little below the hinge, rapidly* narrowing to the front. Other 

 specimens of similar age are broadly ovate or oval, the dorsal valve 

 depressed-convex. As the shell becomes older, the convexity increases ; 

 though .we meet with specimens of an inch and a half in length and 

 width, where the depth of the two valves is but three-fourths of an inch ; 

 and in one specimen of a dorsal valve three inches and a half long, the 

 depth has been but about five-eighths of an inch. In a very symmetrical 

 specimen of the ventral valve about three inches long, the width is half 

 as great, and the depth five-eighths of an inch. 



This shell sometimes reaches a length of nearly four inches ; though the pre- 

 vailing size is from two and a half to three inches in length. It is frequently 

 crushed, and well-preserved, entire and symmetrical specimens are very rare. 



A specimen from Michigan, received from Dr. C. Rominger, and one collected at 

 Mackinac from the same geological horizon, are strongly wrinkled concentrically, 

 but marked with similar i-adiatiug stria which are obscurely preserved. 



The species described under the name Meganteris svblrigonaUs ( Tenth Report 

 on the State Cabinet, page 123, 1857) appears to be only a variety of this spe- 

 cies, and, if found of sufficient constancy, might be thus indicated. 



Geological foiinations and localities. This species occurs in the Schoharie grit, 

 where it is recognized by casts of the interiors of the valves. It is found in the 

 Upper Helderberg limestones at many localities between the Hudson and Niagara 

 rivers, and in Canada West. The principal localities from which I have specimens 

 before me, are Vienna, Ontario county ; Caledonia, Livingston county ; Clarence 

 hollow and Williamsville, Erie county. It occurs in Albany and Schoharie coun- 

 ties, along the outcrop of the Upper Helderberg limestone, and at Cherry-valley, 

 Otsego county, and Bridgewater, €fneida county, New-York. 



' Auipliigeiiia eloiigata, var. uiidulata. 



PLATE LIX. 



Shell smaller than the preceding ; ventral valve very gibbous, and 

 marked by strong concentric undulations, and obscurely by fine radia- 

 ting striae. 

 The specimens are apparently old shells and have a length of only 



about an inch. 



It is desirable to compare a larger number of individuals, before this form can 



be entitled to specific distinction. 



Geological formation and localities. In the Upper Helderberg limestone, at 



Mackinac, and from Michigan : the latter received from Dr. C. RtE>UNOER. 



