PALAEONTOLOGY OF NEW-YORK. 
attaining the dimensions which it has in the limestone below, it possesses 
nevertheless all the essential characters of that one in its symmetrical 
form, and can also be traced into the unsymmetrical forms where the 
area is unequal and the beak distorted, but where no two individuals are 
precisely alike. From these unsymmetrical forms which I have termed 
S. ( Orthisina ) arctostriata, to those where the striae are more unequal, the 
gradation is imperceptible ; and the S. ( 0.) alternata, in its typical form, 
is not more abundant than are the intermediate varieties. 
In the Chemung group, we have a much greater degree of variation 
among those which I have referred to the same species. The symmetrical 
forms, like S. pandora of the Corniferous limestone, can be traced every¬ 
where ; and in the arenaceous beds, they sometimes so much resemble 
those of the Schoharie grit, that specimens laid together might not be 
readily separated. We find among these a greater or less degree of pro¬ 
minence in the muscular impressions, or of the ridge dividing the mu¬ 
scular impressions, and also of the cardinal process, which may have a 
greater or less extension, and the small ridge between the two branches 
of this process may be prominent or otherwise. From these forms there 
is a very simple gradation to such as S. bifurcatus , where the strise are a 
little stronger and more regularly bifurcating : these are often nearly, 
but rarely quite, symmetrical. The type of S. pectinacea is readily reached 
by a partial suppression of some of the striae and a greater development 
of others ; leaving the larger and more prominent ones with two, three or 
more smaller ones between them. 
Such forms as S. arctostriatus have the striae nearly equal, and quite 
thin and sharply prominent; but even in these there are finer inter¬ 
mediate striae, but the interspaces are so nearly equal that the surface 
presents a pretty uniform aspect, though there are many gradations. 
Specimens of this kind have usually broad convex dorsal valves, which 
are sometimes depressed in the centre. 
In the form of S. pectinacea, we have a near approach to the Orthis in- 
equalis of the sandstones beneath the Burlington limestone in Iowa and 
elsewhere. The Burlington specimens are usually very symmetrical; but 
