ADDENDA. 415 



Centre, Medina county, Ohio, in rocks pertaining to the Waverly group. 

 The specimens present no characteristic differences from those of the Lower 

 Helderberg group, or any modifications which cannot be detected among 

 the collections of specimens from that group. Individuals may be selected 

 from any of the formations that will show strong and well marked differ- 

 ences, either from these under consideration, or from each other, but a 

 series connecting the extremes from either the Niagara or Lower Helderberg 

 groups will contain individuals having the same characters with those pre- 

 sented by the Ohio specimens. x 



The occurrence of this species in this Geological position, is the more 

 interesting since thus far, it has not been found in the Hamilton or Che- 

 mung groups of New York. 



Sti'opliodonta siibdeiuissa. 



PLATE XVII, FIGS. 2 A, 2 i. 



Slrophodonta lubdemitta : Hall, Tenth Report on the State Cabinet, p. 145. 

 Strophodonta demista : Hall, Geological Report of Iowa, Vol. I, part 2, p. 495, pi. 5, a, b, c. d 

 Strophodonta dimota ? [demissa] Owen. Geol. Survey of Wisconsin, Iowa, Minnesota. Explana- 

 tion of Table III a, flg. 14. 



Shell semicircular in outline, with more or less extended and soifietimes 



mucronate cardinal extremities. 

 Ventral valve depressed convex, somewhat regularly arching from beak to 



base ; area narrow, crenulated. 

 Dorsal valve concave, nearly following the curvature of the ventral valve ; 



area linear, crenulated. 

 Surface marked by strong radiating striae, which are increased by intersti- 

 tial additions becoming finer and more numerous near the margin in adult 

 individuals. The striae are themselves longitudinally striate on well pre- 

 served specimens, having from three to six striae on each of the larger 

 ones, near the middle portions of the ventral valve. 



I have hesitated to continue this as a distinct species, and have so indicated on* 

 page 114 of this volume. The Western specimens, however, show a constant dif- 

 ference in form and in certain peculiarities of the stri^. While the New York 



