UTICA SLATE AND HUDSON-RIVER GROUP. 295 
surface marked by concentric undulations; muscular impression distinct, close to the 
anterior extremity. 
This fossil presents considerable variation in form, which has given rise to the establish- 
ment of several species, founded either upon natural or accidental characters. The name 
of Pterinea modiolaris has priority ; and since the species possesses all the essential characters 
of the Genus Moprotopsis, I have adopted it under that name, giving the others as syno- 
nymes, with illustrations of the forms on which they are founded. The more extreme 
forms might be regarded as distinct, did we not find numerous intermediate ones, showing 
a gradation from one to the other. The shell is more or less convex, depending on pressure, 
which sometimes obliterates the prominent oblique elevation extending backwards from 
the beak. Owing to the same cause, also, the beak is more or less prominent ; and pressure 
in different directions changes the form of the shell. 
Fig. 1 a. A specimen preserving the two valves in connection, showing the hinge line, and the muscular 
impression, which produces an elevation upon the outer surface of the shell. This is one of 
the most perfect forms, and the one to which Mr. Conrap applied the name of modiolaris. 
Fig. 1 6. Another specimen preserving the two valves, which are laterally compressed. The anterior 
extremities are narrowed, and somewhat obtusely pointed. This is the original of C. an- 
gustifrons of Conrapd; but since there are several intermediate forms allying it to the 
previous figure, I have no hesitation in uniting the two as identical. 
Fig. 1c. A single valve, with the posterior extremity more regularly rounded, and the hinge line 
curved. The shell is less compressed than fig. 1 a, though in other respects similar. 
Fig. 1 d. A smaller and shorter specimen, having a form similar to 1 a, with the base not contracted 
or arched. There are other forms intermediate between this one and fig. | a. 
Fig. 1 e. A small specimen, contracted at the anterior, and expanded towards the posterior extremity. 
The base is arched, and the hinge line apparently curved. This is the form of C. ovata of 
Mr. Conrap, the original remaining in the State Collection. It is not difficult to trace this 
form through intermediate grades till it becomes the unequivocal M. modiolaris. 
Fig. 1 f, A specimen from the shales at Madison (Indiana), preserving both valves. The one shown in 
the figure is intermediate in form between fig. 1 a and 1 c, while the other valve is more 
contracted anteriorly than fig. 1 6. This figure represents the shell contracted below the 
beak, leaving the anterior extremity as an obscure lobe. This character is frequently 
observed in western specimens, while it is less frequent in those of New-York. 
Fig. 1 g. The anterior extremity of a cast of this species, showing the prominence of the muscular 
impression. 
Plate lxxxii, fig. 1. A young specimen of the same species. 
Numerous other forms might be given, showing the variable character of the species, 
but those figured are the prevailing varieties. Several hundred specimens have been 
examined in order to determine the unity of the several species cited. 
Position and locality. This fossil is everywhere found in the central and higher part of 
the group, varying its form as it occurs in arenaceous or argillaceous strata, or from 
compression. It occurs at Turin, Boonville and Martinsburgh, Lewis county; Loraine, 
Jefferson county ; Pulaski, Washingtonville and other places in Oswego county ; and near 
