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. 

 SuEFACE 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 St 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 surfece 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. 

 Pig. 1 y. The surface where the granules aire worn off, and the lamellse are seen to be finely 



striated. 

 Fig. 1 2. Profile of the imbricating concentric striaB. 



Geological position and locality. In the shaly limestone of the Lower Helderberg 

 group : Helderberg mountains; Schoharie, Catskill, Hudson, etc. 



