496 PALiEONTOLOGY OP NEW-YORK. 



plication becomes developed in the bottom of it, and each of the bor- 

 dering plications is dichotomized ; the central one dichotomizing once, 

 and in old shells twice, before reaching the margin. Very young shells 

 show a sinus with a simple plication in the bottom. The mesial eleva- 

 tion of the dorsal valve is simple in very young shells, showing first a 

 central groove, then each marginal plication becomes dichotomized, 

 and at the same time a central plication rises in the median groove ; 

 and the mesial fold, at its base, consists of five distinct plications, the 

 result of the dichotomizing of a single one at the apex. Surface marked 

 by fine concentric striae and stronger imbricating lines of growth. 



Jn the casts of the interior, this fossil presents considerable variety of 

 appearance, owing to the variable extent of the muscular area, the de- 

 velopment of its markings, and depth of the cavity beneath the beak ; 

 characters due in part to the different ages of the shell, but often ap- 

 parently to other causes. 



This is a beautiful and interesting species of Spirifer, being the first one in the 

 order of time, so far as known, in our rocks, which shows the dichotomizing of the 

 plications; and this feature extends only to the mesial sinus and elevation. 



In the Oriskany sandstone in New- York the shell is rarely well preserved, being 

 more or less exfoliated in separating from the matrix, or occurring in the condition 

 of casts, of which gpeat numbers are found in the rock. 



The specimeps figured on Plates xcvin and xcix are all from the sandstone in 

 New-York; while those on Plate c are from the Oriskany sandstone in Maryland, 

 and were obtained only long after the other plates were completed. These are so 

 well preserved, and show the characters so perfectly, that they form a necessary 

 part of the illustration of the species. 



PLATE XCVIir. 



Fig. 1 & 2. Two views from the same specimen, wliich is slightly distorted and the shell 

 partially removed. 



Pig. 8. Ventral valve of a large specimen. 



Fig. 4. A dorsal view, showing the area and foramen. 



Fig. 6. View of a ventral valve. 



Fig. 6. The dorsal side of a specimen where the shell is worn from the mesial fold, and 

 partially from the sides of the valve. 



Fig. 6 a, b. Front and profile views of the same specimen, the ventral side being a cast of 

 the interior. 



