FOSSILS OF THE HUDSON EIVER GEOUP. 71 



convex, most prominent near the umbo; lower valve flat, transversely 

 elliptical and strongly notched on the posterior margin, the notch ex- 

 tending two-thirds the distance from the edge to the center of the valve." 



"Surface marked by distant, radiating, impressed striae, which are in- 

 dented along the bottom by distinct puncta. In exfoliated specimens 

 neither the striae nor puncta are visible, these being features of the ex- 

 terior layer only." 



" The puncta and distant striae are features which distinguish this spe- 

 cies from any other described." 



There are several individuals of this species in the collection of Prof. 

 J. Hall, from Cincinnati, Ohio, and a single lower valve has been noticed 

 among collections received from U. P. James, Esq., of Cincinnati. The Ohio 

 shells differ from the originals of the species (from the hydraulic lime- 

 stones at Clifton, Tennessee, supposed to belong to the Trenton lime- 

 stones) in being perhaps a little more circular in outline, or a little more 

 elongate, and in having the notch of the lower valve much narrower. 

 As the Ohio specimens are all either flattened or otherwise distorted, 

 these differences may not hold good in more perfect individuals ; we 

 therefore prefer to consider them as belonging to the same species ; espe- 

 cially as the structure of the shell and the lines of puncta are, so far as 

 can be determined, perfectly identical. 



Formation and locality: In the shales of the Hudson River group, at Cincinnati, 

 Ohio. 



Genus SCHIZOCRANIA (new gen.). 



Among the unarticulated brachiopods of the Silurian formation there 

 is a group of radiatingly striated shells, which may be typified by Or- 

 hieula ? filosa, Hall, Pal., N. Y., Vol. I., p. 99, pi. xxx., fig. 9, which have 

 usually been referred to the genera Discina and Trematis, but which do 

 net exactly correspond in character to either of those genera. They 

 differ from Discina, and also from Orbiculoidea (to which many of our 

 Palaeozoic Discinoid shells probably belong) in several particulars ; 

 among which may be mentioned the striated surface, the marginal beak 

 of the upper valve, and the widely divided posterior margin of the lower 

 valve, which forms a deep marginal notch, instead of an eccentric fora- 

 men or perforation, as in those genera. The lower valve in this group is 

 firmly fixed and cemented to foreign substances, while those of the above 

 genera are free, the whole being attached only by a byssus passing 

 through the foramen of the lower valve. The substance of the shell is 

 also of a much more calcareous nature, if not entirely so. To the genus 



