Cincinnatian and Lexington Fossils 53 



the ventral valve. jNIoreover, in the typical forms, the primary pli- 

 cations are distinctly coarser along- the posterior half of the shell. 

 Owing to bifurcation, these plications may appear less coarse near 

 the anterior margin of the shell. The small specimen represented 

 by figs. 2 a, g, i, n, on plate 32 B, of the Xezo York Paleontology, 

 vol. I, is too small to show the characteristics of the species very 

 well, and the originals of 2b and c can not be identified. On this 

 account, the original of figures 2 d, h, k, and o have been selected 

 for illustration in this paper. 



Specimens of this type occur throughout the Richmond, and 

 are especially common in the upper part of the Waynes vi lie bed, but 

 forms identical in character appear as low as the Bellevue, and in the 

 upper Fairmount. The types are labelled as coming from Cincin- 

 nati, Ohio, but their original source and horizon is not definitely 

 known. 



In the original description it is stated that Heberfella sinuata 

 occurs at Maysville, Kentucky, and at Cincinnati, Ohio, but that it 

 is less frequently found at Oxford, Ohio, and at Madison, Indiana. 

 This favors the Maysville formation as the source of the types. 



Since the median depression near the beak of the brachial valve 

 usually is too faint to be readily seen, and since it occurs both on 

 shells having plications of medium size and on those having coarse 

 plications, it appears doubtful whether the absence of this slight 

 depression is sufficient to give name to a separate variety, especially 

 in the Maysville representatives, although in the upper Waynesville 

 and in the Liberty this feature becomes more constant. Moreover, 

 it is difficult to differentiate between Hebcrtella sinuata and Heber- 

 tella occidentalis on the basis of the coarseness of the primary pli- 

 cations, except in the case of selected specimens. At the same 

 locality all the intermediate stages are almost certain to be found. 

 However, the discriminations proposed by Hall are of interest. 

 These forms of Hcbcvtella belong to what might be called a series 

 of nascent species, not yet fully differentiated, corresponding to 

 the numerous variations in the genera Platystrophia, Plectorthis, 

 Cyclocoelia, and Rafinesquina during Maysville time. 



Hebertella occidentalis, Hall. 



(Plate II, figs, i, 2.) 



The chief characteristic of Hebertella occidentalis is the slight 

 median depression near the beak of the brachial valve, which disap- 



