LAMELLIBRANCHIATA. 511 



Modiolopsis exoellens 1 



strongly elevated filling of the anterior adductor scar. The latter is large, of oval 

 shape, horizontally marked in its upper half, sharply defined all around and, because 

 of the brevity of the anterior end, is situated partly beneath the point of the beak. 

 Posterior scar large, but so faintly impressed that its exact shape cannot be deter- 

 mined with the material at hand. Pallial line distinct only in the anterior half, 

 where it consists of an obscurely pustulose raised line. 



To this species I refer provisionally a badly distorted mould of the exterior of a 

 right valve, collected by me at Spring Valley in 1887. Its surface is marked 

 precisely as described above, but the reference is still rendered doubtful by the fact 

 that its anterior end is a little longer than is normal for the species. There is, how- 

 ever, no reason to doubt that M. concentrica occurs in Fillmore county, and it is to 

 draw attention to its probable occurrence in Minnesota that the species has been 

 included in the^ report. 



Formation and locality. — A common species in the upper beds of the Cincinnati group at numerous 

 localities in Ohio, Indiana and Kentucky. Probably also in the Hudson River shales near Spring Valley, 

 Minnesota. 



Modiolopsis exoellens, n. sp. 



PLATE XXXVI, FIGS. 13-14. 



This species, of which we have five specimens, is closely related to M. concentrica 

 Hall and Whitfield, and was at first confounded with it. A careful comparison 

 however proved its distinctness in the following respects: It attains a larger size, 

 the casts are more uniformly convex, with the mesial sulcus, on both the shell and 

 the cast, much shallower, for which reason the ventral margin is very slightly 

 convex where it is sinuate in that species. The outline differs also in the postero- 

 cardinal region being less uniformly curved and more prominent at the extremity 

 of the hinge. The anterior en'd is longer so that a line drawn from the point of the 

 beak across the shell at right angles to the hinge line passes within the inner border 

 of the anterior adductor scar, whereas it cuts a third of the scar away in M. concen- 

 trica. Finally, the concentric surface markings are finer and the difference between 

 them as developed on the cardinal sldpes and on the sides of the shell is a much 

 less striking feature. The number of the concentric lines at a point about midway 

 between the beaks and the posterior extremity varies in different specimens from 

 six to nine in 5 mm. 



What I regard as a nearer ally occurs at the top of the Cincinnati hills. The 

 outline of this species is intermediate between figures 6 and 13 of plate xxxvi. In 

 its characters also it approaches one almost as nearly as the other. 



Formation and locality. — Upper part of the Hudson River group, Spring Valley and Granger, 

 Minnesota. 



MvA. Reg. No. 8374. 



