DESCRIPTION OF SPECIES 203 



and a number of less perfect individuals are among the collections 

 studied; no essential divergences from the form as it ordinarily occurs 

 were noted. 



Horizon and locality. Lower Mercer limestone : Perry County, 

 Locality 35, r; Muskingum County, Localities 43, 45, r; Licking County, 

 47, 49, r. McArthur limestone: Vinton County, Locality 84, r. 



Nucula subrotundata Girty mss. 

 PI. X, figs. 14-16 



Description. A species of Nucula from the Sharon ore is identical 

 with one described and figured in manuscript by Girty from the Morrow 

 formation of Arkansas. Girty 's manuscript description is quoted below: 



"Shell small, subtriangular to subcircular in outline. In shape 

 all three sides are gently convex, the anterior side most and the posterior 

 side least. In length the inferior side is greatest and the posterior side 

 is considerably the shortest. The inferior-posterior angle is rather 

 narrowly, the inferior-anterior angle rather broadly, rounded. In a 

 genus in which so many of the species are strongly convex the convexity 

 of N '. subrotundata is generally rather low, though it varies greatly in 

 different specimens (partly due to the degree of maturity), and it may 

 be rather high. The chief flexures occur near the anterior and posterior 

 margins along lines that make with each other an angle rather less than 

 a right angle. A projection of the inflected parts beyond these angles 

 gives the sides their curved outlines. The oblique anterior outline is 

 thus rather regularly curved; the posterior outline may be essentially 

 straight. On the posterior side a 'lunule' is more or less faintly out- 

 lined by a sulcus which tends to produce a slight emargination in the 

 outline. A few specimens referred under this species have the lunule' 

 rather strongly defined in this way. The beaks are not very prominent. 

 They are rather attenuated and are conspicuously turned backward. 



"The shell is, for its size, very thick and massive, and it is marked 

 superficially by rather coarse, strong and regular concentric striae." 



Remarks. This species of Nucula is very abundant in the Sharon 

 and less so in the Harrison ore, but as far as is known it is confined in 

 its occurrence in this State to these two horizons. It has, moreover, 

 been found only at a single locality in each member, and only in the 

 form of internal casts. The Ohio forms differ from those of the Morrow 

 formation of Arkansas in being on the average a little more gibbous. 

 In the Sharon ore this species of Nucula is associated with N. lunulata 

 Girty mss. as in the Morrow formation, and also with N. elongata n. sp. 



Horizon and locality. Harrison ore: Jackson County, Locality 1, 

 c. Sharon ore: Scioto County, Lick Run, Locality 2, a. 



