DESCRIPTION OF SPECIES 237 



the anterior part where they are larger and less crowded, the concentric 

 arrangement is most prominent, while posteriorly where they are ex- 

 tremely minute, the radial arrangement is most prominent, giving the 

 appearance of very fine radiating lines. 



Dimensions. A well preserved right valve of large size from the 

 Lower Mercer limestone at Flint Ridge measures: length 25 mm., 

 height just behind beak 10 mm., convexity 2.5 mm. Two right valves 

 of average size from the McArthur limestone of Vinton County meas- 

 ure: length 22 mm., 17 mm.; height 9.5 mm., 7 mm.; convexity 2 mm., 

 2 mm. 



Remarks. The most important characters of this form are the 

 peculiar surface markings which are excellently shown on external 

 molds, the large number of radiating costae, the fine regular con- 

 centric lines, and especially the numerous, fine spinules covering the 

 entire surface. In general contour and dentition, as far as can be de- 

 termined, these shells belong to the genus Pleurophorus, although the 

 spinulose sculpture has not been credited to the genus. It has, how- 

 ever, been commonly observed on representatives of Sedgwickia and 

 Allerisma as well as on certain forms of Pleurophorella. 



Among the described species of Pleurophorus, P. subcostatus, P. 

 immaturus, and P. occidentalis approach this form most closely in 

 general contour and appearance, although the surface markings de- 

 scribed above have apparently not been heretofore observed. The 

 species under discussion is abundant in the Lower Mercer and McArthur 

 limestones, and is probably the same form which Herrick referred to 

 as P. subcostatus ? in his study of the Lower Mercer fossils of Flint 

 Ridge. Another smaller, more compact form from the same locality 

 has been given the name P. immaturus by the same author with the 

 suggestion that it may be the young of P. subcostatus^. Specimens 

 intermediate between the small compact form and the large one have 

 been observed in the collections studied, and as the differences between 

 the two foims are those which one would naturally expect to find be- 

 tween the mature and young of a given species, it is the conclusion of 

 the writer that P. immaturus actually represents the same form as 

 that cited by Herrick as P. subcostatus^. 



However, whether Herrick's P. subcostatus ? is really the same 

 species as that described by Meek and Worthen as P. subcostatus may 

 be questioned. Typical P. subcostatus is described as having the cardi- 

 nal and basal margins subparallel, with the basal margin sinuous, and 

 judging from the figures, 1 the anterior extremity is comparatively 

 prominent. P. occidentalis Meek and Hayden, another species very 

 closely related to P. subcostatus , is described as having the cardinal 

 and basal margins either subparallel or converging, although as figured 



*Meek and Worthen, Geol. Surv. 111., Vol. 2, PI. 27, Figs. 2, 2a, 1866. 



