ORTHIDES OF THE HAMILTON GROUP. 4S 



front of the dorsal valve nearly parallel to each other, or slightly converging ; 

 while those of the present species, as well as of 0. multistriata, are divergent. 



Geological formation and locality. Limestone of the Upper Helderberg group : 

 New- York and Ohio. 



ORTHIDES OF THE HAMILTON GROUP. 



The Hamilton group, throughout the extent of its calcareous shales, is 

 every where marked by the presence of one or more species of Orthis. 

 The O. vanuxemi is the most abundant, while two allied species are of 

 frequent occurrence. In the more eastern localities in the State of New- 

 York these fossils are less common, and are usually in the condition of 

 casts of the interior; affording, however, specimens not less instructive 

 than those where the shells are entire. 



In some localities in Western New- York, the prevailing species often 

 occur in a profusion equal to the more common forms in the Lower Hel- 

 derberg group in Eastern New-York. 



A. Species of the type of Orthis elegantula, Orthis perelegans, etc., with the ventral 



valve nearly flat or depressed-convex. 



Orthis lepidus, Okthis solitaria. 



Ortliis solitaria. 



PLATE VI. 



Orthit tolUaria : Hah, Thirteenth Report on the State Cabinet, 1860, p. 60. 

 Compare Orthi$ lenticularu, Yahuxex, page 8S, this volume. 



Shell subcircular, a little transverse : valves unequally convex ; hinge- 

 line about two-thirds the greatest width of the shell. Ventral valve 

 highly convex in the middle and above : beak small, prominent, slight- 

 ly incurved ; area low and well defined. Dorsal valve depressed-convex, 

 most prominent near the umbo, with a broad shallow mesial sinus at 

 the front, which rarely extends beyond the middle of the valve : beak 

 small, not prominent ; area linear. 



Surface marked by fine radiating bifurcating striae and strong concentric 

 lines of growth. 



