DISCING OF THE HAMILTON GROUP. * 91 



Surface finely and evenly striated by fine regular elevated striae, distant 

 from each other more than twice their width. The apex of the dorsal 

 valve is about one-third, and sometimes less than one-third the length 

 of the shell from the posterior margin. Perforation of the ventral valve 

 narrowly oval or sublinear, about one-third the length of the shell from 

 the posterior margin, and extending .towards the edge of the shell. 

 This species resembles in form the D. lodensis of the Genesee slate ; but it is 

 more coarsely and distantly striated, and does not present the radiating folds or 

 undulations observed in that species. The position and relations of the apex and 

 foramen are similar to those of the D. lodensis; and it is possible that conditions 

 of the sediment, and other physical causes, may have affected the external cha- 

 racters, and that we have only a well-marked variety of that species. The species 

 is not uncommon in the Hamilton group, both in the bluish shales and in the 

 dark slaty beds, which are of the same character as the Genesee slate. 



A single specimen of a ventral valve, found in the upper part of the Chemung 

 group in Steuben county, possesses characters so entirely similar to those of the 

 Hamilton group, that I can find no means of separating it. 



The oval form and excentric position of the apex are suiEcient to distinguish 

 the species from others of these groups. The elliptical specimens measure half 

 an inch, or more, in length, with a width of nine-twentieths of an inch. 



Geological formation and locality. In the Hamilton group, on the shores of 

 Seneca lake near Ovid, and in the upper part of the group on the shores of the 

 Canandaigua lake ; and in the green shale of the Chemung group in the south 

 part of Steuben county, at Troupsburgh. 



Discina (sp.?). 



PLATE n. 



A small nearly circular specimen of a ventral valve, occurring in the 

 Marcellus slate, is marked by fine crowded and scarcely distinct striae : 

 the apex is excentric, being scarcely more than one-third the length of 

 the shell from the posterior margin ; with a short, narrowly oval 

 foramen. 

 This specimen is unlike any other one before me, and maybe a distinct species; 



but I hesitate to designate it without more material. 



Geological formation and locality. In the Marcellus slate : near Bridgewater , 



N.York. 



