Ciuciiinafiau and Lexington Fossils 47 



is not clean. A cercain quantity of foreign matter attached to 

 the shell gave rise, in the original description, to the statement that 

 the strong radiating plications are marked as if by the bases of 

 short spines, or of sqnamose projections of die shell, which, in 

 perfect specimens, indicate a character much like Atrypa as per a. 

 This was a most unfortunate comparison, and readily accounts for 

 the failure readily to identify this shell, once it had become sepa- 

 rated from its original label. No other shell occurs in the Hall 

 collection which even approximately resembles the outline of his 

 Plcctorthis cquivalvis. 



Plccforfhis cqiiivahis belongs to one extreme of a group of 

 shells which is very abundant in the Fairmount bed at Cincinnati, 

 and \icinii;y. Usually the width is distinctly greater than the 

 length, in the ratio of 10 to 8 or 7 (plate VI, figs. 3, 2). The 

 tendency toward a trifid arrangement of plications is marked, but 

 frequently only one of the secondary plications is present between 

 two primary plications, and the term bifid, used in Hall's original 

 description, becomes appropriate. Toward the anterior margin of 

 the sliell, the secondary plications are less conspicuously depressed 

 below the level of the primary ones than in Plcctorthis fissicosfa, 

 and this is their chief difference. 



In a large suite of specimens from the Fairmount at Cincin- 

 nati, Ohio, considerable variation in the relative width of the shell 

 may be noticed. Narrow individuals occur, but there appears to 

 be no evidence that these occur regularly enough to be considered 

 a variety. Apparently the narrow form selected as the type of 

 Plcctorthis cqitiz'ahis by Hall is representative only of individual 

 variation, not of group variation. It is merely an individual speci- 

 men which has not grown to its full size laterally. Specimens of 

 this type are never found in numbers at some one locality or 

 horizon, but singly and widely separated. It is only when speci- 

 mens from widely diiTferent localities and from different horizons 

 are selected and brought together that the impression can be pro- 

 duced that there is a variety in which the shell is narrow. With 

 the exception of very few specimens, the species proper consists of 

 broad, not of narrow shells, and to these broad shells the name 

 Plcctorthis cquivalvis should be applied even if the original type 

 specimen was a narrow form. 



It is needless at this late dace to state that Plcctorthis cqui- 

 valvis is not present in any ])art of the Trenton of New York or 

 elsewhere. 



