f4 PALEONTOLOGY OF NEW-YORK. 



Among a large number, the gradations to S. bifurcata, where the striae are 

 regularly bifurcating and of nearly equal size, are almost imperceptible. 

 It is apparently as the individuals become larger that there is an increase 

 in the gibbosity of the dorsal valve, while at the same time the striae 

 become somewhat regularly alternating in size, and in some specimens 

 nearly equal, but retaining a sharpness of outline not observed in 

 specimens from the limestone. 



It was to those specimens with sharp equal striae and gibbous dorsal valves that 

 I originally gave the name Strophomma arctostriata ( Report of Fourth District ). 

 Specimens of this kind approach, both in form and expression, the figures of 

 Streptorhynckus crenistria, illustrated by Mr. Davidson ( Monograph of British 

 Carboniferous Brachiopoda), where figs. 2 & 8, Plate xxvii, may be compared with 

 figs. 8, 11 «fe 14 of Plate x of this volume. The British Carboniferous specimens 

 are larger than ours of the Chemung group. 



The gradations of form and character are pretty well shown on Plate x. Figs. 

 1-6 are of the smaller forms, having the prevailing surface-marking chanicteristic 

 of S. pectinacea- The ventral valve fig. 2, and the cast of ventral and dorsal valves 

 respectively figs. 3 & 5, are more like S. bifurcata. 



In figs. 7 & 8 we have gibbous dorsal valves, somewhat depressed towards the 

 front, and with the striae subequal or alternating ; and fig. 10 is the ventral valve 

 of similar character. 



In the figs. 11 - 17, we have specimens with somewhat unequally convex dorsal 

 valves : the ventral valves are irregular towards the umbo, and the beaks more 

 or less distorted ; while the area presents much variety in its degree of elevation. 



In fig. 18, we have a ventral valve with the striae as regular and even as those 

 from the limestone ; and in the casts, figs. 19 - 23, we have symmetrical forms with 

 oven striae, and with form of muscular impression undistinguishable from those of 

 the Schoharie grit, the only distinctive feature in those from the Chemung group 

 being the tenuity of the shell. 



The varieties illustrated on Plate x are common and often abundant in the 

 Chemung group, having a considerable vertical range, and in horizontal range are 

 almost coextensive with the formation within the limits of the State. They are 

 most abundant and present greatest variety of form in Chemung, Allegany and 

 Cattaraugus counties. 



Following the comparison of the New- York Chemung species with similar forms 

 of the Waverly sandstone* of Ohio (which I have regarded as of the same age), 



• The term " tubcarboniferout" haH been applied by gomo authors to these sandstones. This is a very 

 pemicioui nomenclatare, as the terra is equally applicable to all rocks below the Carboniferous period. 



