CLINTON GROUP. 
73 
On its first appearance in the Clinton group, it shows its variable character in a remarkable 
degree, and it is scarcely possible to avoid referring the individuals to distinct species. In many 
of the young specimens, the ventral valve is nearly flat, or slightly convex, with a depression 
along the centre from beak to base. In specimens of medium size the valves are nearly 
equal, and in older ones the ventral valve is the more convex. Again there are others where, 
in the young shell the ventral valve has no depression in the centre, and is equally convex 
with the dorsal valve. In the radiating striae or plications it is equally variable; many speci¬ 
mens having them very distinctly dichotomous, while others are nearly undivided from the 
beak. In many young shells the concentric striae leave the plications nodulose at their crossing; 
while there are specimens having the plications quite free from such characters, and entirely 
smooth. 
Notwithstanding all these differences,-however, there seems no possibility of drawing a line 
of separation between species, and I have therefore given the principal varieties of form under 
the same name. It never reaches the maximum size in this geological position ; and though 
appearing in the Niagara group under one of its many variable forms, and again in the Pen- 
tamerus and Delthyris shaly limestones, it only reaches its greatest of size in the Onondaga 
limestone and Hamilton group, where it occurs in great numbers. Specimens of the form pe¬ 
culiar to these different geological positions will be given in the succeeding plates. 
Fig. 8 a. A specimen of the ordinary size of this species in the Clinton group. 
Fig. 8 b, c. Ventral and front view of a young specimen, in which the valves are equally convex, 
and the concentric laminae very rugose. 
Fig. 8 d, e. Dorsal valve and profile view of an older specimen, with the valves almost equally 
convex. The concentric laminae are less prominent than in the preceding specimen. 
Fig. 8 f, g. Ventral valve and cardinal view of a specimen slightly larger than the preceding, 
in which the dorsal valve is more convex, with a slight depression in the ventral 
valve, reaching from the beak halfway to the base. 
Fig. 8 h. The dorsal valve of a specimen where the shell is partially exfoliated. The plications 
are flat, broad and scarcely dichotomous, extending to the beak undivided. 
Fig. 8 i. The cast of the dorsal valve of a similar form, where the plications are flat and un¬ 
divided. 
Fig. 8 k, l. The ventral and dorsal valves of a young specimen, where the ventral valve is flatter 
than the dorsal, and has a distinct mesial depression. 
Fig. 8 m, rt. Front and profile view of a similar form, showing the inequality of the valves and 
the mesial depression. The strias in these forms are all dichotomous. 
Position and locality. This species occurs in the lower green shale at Sodus, in the Penta- 
merus limestone of the group, and in the upper shale and limestone, presenting -much variety 
of form and appearance. ( state Collection.) 
[ Palaeontology — Vol. ii.] 
10 
