May, 1919] Silurian Fossils From Ohio ZT7 



specimens from the same locality and horizon are present, 

 preventing the discrimination of these forms into separate 

 so-called species. In the absence of abundant material the more 

 extreme forms might appear distinct enough to be separated 

 at least as varieties, if not as species. 



Stropheodonta (Brachyprion) plana Foerste. 



Plate XVI, Figs. 2 A, B. 



1909. Stropheodonta {Brachyprion) planus Foerste, Cincinnati Soc. Nat. Hist., 

 Jour., vol. 21, P. 22, pi. 1, figs. 13 A-C; pi. 2, figs. 11 A, B. 



At the Whitfieldella cylindrica horizon, in the Lower or 

 Bisher member of the West Union formation at Hillsboro and 

 Danville, Ohio, several specimens of Stropheodonta occur which 

 differ sufficiently among each other to suggest the presence of 

 more than one species, but the material at hand does not permit 

 of definite diagno.sis. The most obvious difference is that some 

 specimens appear shorter and broader than others, the ratio 

 of length to width in the former specimens being as 7 to 10, 

 in contrast with the ratio of 8 to 10 in the latter; but, apparently, 

 specimens of intermediate length occur also. Some of the 

 specimens are more convex than others, and in these more 

 convex specimens the radiating striations are subequal in size, 

 while in several of the flatter specimens the radiating striations 

 are finer and every fourth one tends to be more prominent; 

 however, there is considerable variation in convexity and 

 variation in the character of the radiating striations, and such 

 as those noted above are known to be not uncommon among 

 the Strophomenidae. For the present, therefore, the specimen 

 here figured will be referred to the species already described. 



Compared with other shorter forms from the same localit}' and 

 horizon, it is more convex and its radiating striations are more nearly 

 subequal in size. The shell material is exfoliated, so as to present an 

 almost perfect cast of the interior of the pedicel valve. Postero- 

 laterally the muscular area is clearly delimited, the sides making an 

 angle of SO degrees. Posteriorly, within this area, there are two oblong 

 shallow depressions, separated by a low, sharp, narrow ridge, and the 

 former are interpreted as the anterior adductor scars. The anterior 

 margin of the muscular area is not distinctly delimited in this specimen. 

 Between the postero-lateral margins of the muscular area and the 

 adjacent parts of the hinge-area, there are numerous sharply elevated 

 granules, characterizing the so-called ovarian spaces. (Plate XVI, fig. 2.) 



In an associated brachial valve, in which the ratio of length to width 

 is as 77 to 100, almost the entire interior of the valve is minutely gran- 



