FOSSILS OF THE CLINTON GROUP. 571 



strongly inward only near the beak, and this for such a short distance 

 that the cardinal area remains almost flat. The radiating plications are 

 in general much more numerous and closer, though still presenting the 

 same general characteristics. In this form the finer radiating striae 

 could not be distinctly detected, while the concentric striae of growth 

 were very distinct. If this character should hold good in larger collections, 

 it would indicate another means of differentiating these extreme form's 

 but that seems at present doubtful. Specimens showing these charac- 

 teristics are found at Soldiers' Home Quarry, Ohio, and at Hanover, 

 Indiana. (Plate 31, Figs. 4, 5.) 



Specimens of this type find their nearest relatives, at least as regards 

 external form, in the sub-genus Dinorthis of Hall and Clarke. Moreover 

 in the internal characters there is more resemblance than has perhaps 

 been suspected. At least on cleaning specimens of pedicle valves from 

 Soldiers' Home, Ohio, and Hanover, Indiana, muscular scars were found, 

 which bear considerable resemblance to those of Orthis pectinella except 

 in the character of the adductor muscular impressions. The anterior edge 

 of the muscular area is namely strongly indented, and its anterolateral 

 border is more or less sinuately concave, giving to the middle portion of 

 this area the greater width. The adductor muscular impressions are 

 represented by a narrow median linear groove, bordered on either side 

 by a narrow ridge. This linear groove is traversed at times lengthwise 

 by a fine raised striation. In Dinorthis the adductor muscles are con- 

 fined to the more central portion of the area, and are of an oval-oblong form. 



The muscular impressions of the first described form were not dis- 

 tinct in specimens from these localities, but, as far as could be made out, 

 were of practically the same character. In a former paper a strongly 

 bordered and markedly tripartite muscular area, with a broad central 

 division, was figured from a pedicle valve found at Hanover, Indiana. 

 (Plate 31, Fig. 4.) Its anterior margin was greatly thickened, and from 

 its middle extended a short median ridge, very soon disappearing ante- 

 riorly, or in other words these are characteristics recalling features of the 

 muscular area of the pedicle valves of Orthis fausta and Orthis Dayton 

 ensis. Tne convexity of this valve is moderate and rather equal, 

 though apparently occupying quite an anomalous position in this 

 group of shells; forms with a closely similar structure of the muscular area 

 in the pedicle valve will be described below. 



Orthis flabellites as recently figured by Hall and Clarke, occupies an 

 almost exactly intermediate positon between the extreme forms above 

 described. Its brachial valve is about as convex as that of the second of 

 these forms. Its pedicle valve is however neither as convex nor as 

 much flattened as in the above described extreme forms. The artist 

 evidently found the anterior margin of the muscular area of the pedicle 

 valve of the typical Lockport, N. Y. forms to be indented as in our speci- 

 mens. If it be indeed necessary to distinguish our American forms 



