PALAEONTOLOGY OF NEW-YORK. 
Cyrtia dalmani. 
Plate XXIY. Fig. 1 a - y . 
Shell 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. 
Sukface granulose-punctate. 
This species differs from the Cyrtia ( Spirifer) pyramidalis of the Niagara group ; 
having 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 lamellae 
are stronger in the species under consideration than in the Niagara species. 
I have heretofore 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. 1 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. 
