42© 
PALAEONTOLOGY OF 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. 
In 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 great numbers are found in the rock. 
The specimens figured on Plates xcvm 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 XCVIII. 
Fig. 1 & 2. Two views from the same specimen, which is slightly distorted and the shell 
partially removed. 
Fig. 3. Ventral valve of a large specimen. 
Fig. 4. A dorsal view, showing the area and foramen. 
Fig. 5. 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. 
