UTTCA 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 hais priority; and since the species possesses all the essential characters 

 of the Genus Modiolopsis, 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. Conrad applied the name of modiolaris. 



Fig. 1 b. 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. a?i- 

 guslifrons of Conrad ; but since there are several intermediate forms allying it to the 

 previous figure, I have no hesitation in uniting the two as identical. 



Fig. 1 e. 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 i. 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. 1 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. Conrad, 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 / 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 b. 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 Ixxxii, 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 



