NUCULANA L^VISTRIATA. 207 



Worthen, from the upper part of the St. Louis group of the Carboniferous 

 Limestone Series of Illinois." He did not attempt to point out any details of 

 difference, and, indeed, I am of opinion that the species are identical. On 

 comparison, the descriptions of the two species are found to be very strikingly 

 similar : 



Yoldia Isevistriata. Leda intermedia. 



Shell. — Transversely subovate. Transversely elongate. 



Ventral margin. — Semi-ovate curved, most Convex, most so on the anterior side, 



prominent part being a little in advance of the 

 middle. 



Anterior side. — Eounded. Rounded. 



Posterior side. — Compressed, narrower than Compressed, narrower than the anterior, with 



the other, and narrowly rounded at the ex- blunt rounded termination, 

 tremity. 



Beaks. — Depressed and located centrally. Nearly central, depressed. 



Dorsal outline. — Declining slightly, and some- Slightly convex on the anterior side, concave 



what concave behind the beaks ; a little convex and inclined downwards on the posterior side, 

 in front. 



Surface. — Apparently smooth, but showing Close, regular, concentric striae, 



under a good lens extremely fine, regular, closely 

 arranged, concentric stria?. 



Dimensions. — 6 : 2 : *14 inch. 1 : 3 : "18 inch. 



In their observations on Yoldia? Isevistriata, Meek and Worthen state, " This 

 shell is apparently so very closely allied to L. subscitida of Meek and Hayden 

 from Kansas, that we would scarcely have ventured to regard it as a new species 

 were it not for the widely different geological positions occupied by the two 

 shells. The Kansas species, however, is a little more convex, while its dorsal 

 slope behind the beaks is rather more concave in outline." 



Yoldia Stevensoni, Meek (op. cit.), should probably be added to the synonyms 

 of N. Isevistriata. The descriptions and figures agree very closely, the chief 

 difference noticeable being that the concentric stride are shown in the figure as 

 quite visible to the naked eye. The dimensions (length "7 in., dorso-ventral "3 in., 

 lateral 16 in.) agree exactly with the specimens figured here in PI. XV, figs. 39 — 

 41, from the Collection of Mr. J. Neilson, of Glasgow, obtained from the Lower 

 Limestone series of Brockley. 



