LOWER HELDERBERG ROCKS. 
199 
Spirifer cyclopterus (n.s ). 
Plate XXV. Fig. 1 a - z. 
Shell semicircular : extremities of the hinge line more or less symme¬ 
trically rounded. Ventral valve gibbous : beak elevated, more or less 
incurved; sinus moderately deep, curved on the sides and nearly flat 
in the middle. Dorsal valve very convex towards the middle, the me¬ 
sial fold abruptly elevated and very prominent : beak little elevated 
above the hinge line, and scarcely incurved. Area moderate, scarcely 
extending to the extremity of the hinge line. Foramen large. 
Surface marked by five to seven rounded plications on each side of the 
mesial line, concentrically marked by fine close imbricating lamellose 
striae, which are more or less prominent, depending on the condition 
of preservation in the shell : surface of lamellae ornamented by short 
fine vertical striae or crenulations, which project in fimbriae on the 
edge of the lamellae. 
This species presents comparatively little variety in form; though there are, 
rarely, to be found individuals with the cardinal extremities much extended as in 
fig. 1 g, and others where the extremities are subangular as in fig. 1 /. The greater 
number, however, are rounded as in 1 a - i. 
In its surface characters this species resembles S. crispus of the Niagara group, 
but is much larger, has a greater number of plications, and a narrower and longer 
area; while the valves are more nearly equal in size, and the beak of the ventral 
valve is more elevated. Where the specimens are small, the surface characters alone 
are sufficient to distinguish one from the other. The muscular impressions of the 
ventral valve are very deep and strong, the margins of the foramen terminating in 
prominent teeth. 
This species likewise resembles S. duodenaria of the Upper Helderberg limestone; 
but the latter has a narrower area, a less prominent beak of the ventral valve, and 
a shallow sinus, while the mesial fold of the opposite valve is less prominent. 
Fig. 1 as. Figures of several individuals showing gradations in size, modifications of form,etc. 
Fig. 1 u. Enlargement of surface, where the edges of the lamellae are ornamented with little 
granules, giving the appearance in profile as represented in fig. 1 z. 
Fig. 1 y. The surface where the granules are worn otf, and the lamellae are seen to be finely 
striated. 
Fig. 1 z. Profile of the imbricating concentric striae. 
Geological position and locality. In the shaly limestone of the Lower Helderberg 
group : Helderberg mountains; Schoharie, Catskill, Hudson, etc. 
