90 PALEONTOLOGY OF NEW-YORK. 



single dorsal valve, recognized as of this species, the cardinal process is 



slender, the parts narrowly diverging, and continued in a narrow rounded 



elevation to the middle of the valve. No defined muscular impressions 



are visible in this specimen. 



The figures 9, 10 & 11, Plate xii, and figures 1-5, Plate Xlii, illustrate some of the 

 usual forms and the preceding characters of this species. 



Notwithstanding that these characters can be traced through the various stages 

 of growth offering differences from the ordinary forms of S. inequiradiata, there 

 may still remain some doubt whether a large collection may not show intermediate 

 forms which will unite the two. 



The Lepttma stephani of Barrande has a similar wrinkled surface in some of the 

 individuals, though the intermediate finer striaa are not shown in the figures. In 

 these two shells, the muscular impressions are quite distinctive ; and the figure 7 6, 

 Plate X, Barrande, approaches in character to some varieties of S. inequiradiata. 

 Some varieties of the species under consideration may be compared with S. cor- 

 rugata, Portlock s'p. (not of Conrad). 



The species differs in form of shell, character of striae, and muscular impressions, 

 from the S. inequiradiata. 



Geological formation and locality. Rarely in Schoharie grit, but not uncommon 

 in the Corniferous limestone, in the Helderberg mountains, Schoharie ; in Ontario 

 county; and at Stafford in Genesee caunty and Williamsville in Erie county. It 

 has likewise been recognised among the collections made at the Bake-oven on the 

 Mississippi river, in Illinois. 



Stropliodonta hemisplicrica. 



PLATE XIV. 



Strophomena {Strophodonta) hemitpherica : Hali, in Tenth Report on the State Cabinet, p. 113. 1857. 



Shell subhemispheric, semielliptical in outline : hinge-line equalling or 

 greater than the width of the shell ; cardinal extremities sometimes 

 salient and auriculate ; length and breadth about as four to five : in 

 the larger specimens the length is about two inches, with a breadth 

 of two and a half inches. Ventral vaiae gibbous, the elevation of the 

 curvature being often equal to half the length of the shell, sometimes 

 very abruptly declining to the front : umbo moderately, or sometimes 

 more extremely elevated ; apex incurved ; area narrow ; margin ere- 



