384 
PALAEONTOLOGY 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 symmeti’ical 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 radiating strias which are obscurely preserved. 
The species described under the name Meganteris subtrigonalis (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 foi'mations ancl 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, Oneida county, New-York. 
Aisiphigema elongata, var. umhiiata. 
PLATE LIX. 
✓ 
. Shell smaller than the preceding; ventral valve very gibbous, and 
marked by strong concentric undulations, and obscurely by fine radia¬ 
ting strite. 
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. Rceminger. 
