«06 PALAEONTOLOGY OF NEW-YORK. 



CyrtUi dalmani. 



Plate XXIV. Fio. 1 a - y. 



Shbll trigonal : valves extremely unequal. Ventral valve triangularly 

 pyramidal. Dorsal valve semicircular : mesial lobe flat, or with a 

 slightly depressed line : beak scarcely defined, or rising above the 

 hinge line. Hinge line straight. Area triangular, flat or slightly arcuate. 

 Foramen narrow, linear, usually closed in the lower part, with a semi- 

 tubular opening above : concentric lamellae strong, and often very 

 conspicuous near the margin. 



Surface granulose-punctate. 



This species dififers from the Cyrtia {Spirifer) pyramidalis of the Niagara group ; 

 havu]g the dorsal valve more uniformly convex, the mesial fold broader and more 

 prominent ; while the depressions are not so deep, the mesial sinus is broader, the 

 plications bounding it are less conspicuous, and the concentric imbricating lamellse 

 are stronger in the species under consideration than' in the Niagara species. 



I have heretotbre referred this species, with doubt, to the C. heteroclitus of Europe; 

 for among the variety of forms referred to that species, it is difficult to know the 

 typical one. It differs, however, from the Eifel species of that name. 



The illustrations given present a great variety of form and proportions, so that 

 the extremes might readily be regarded as distinct species; but after the examina- 

 tion of a great number of specimens, I am unable to find any reliable characters 

 for their separation. 



Fig. \ a - y. Illustrations of the ventral, dorsal, profile, and cardinal views of this species, 

 representing the principal varieties of form. 



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

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



