CSVERTEBRATE PALAEONTOLOGY. Ill 



V o 1 tl i a Evansi, M. &H. 



Plate 28, Bgs. 10, a, b, c. 



Nucula Evansi, Meek and Hayden (April, 1850), Proceed. Acad. Nat. Sci. Philad., VIII, 84. 



l.ula Evansi, Meek and Haydeu (May, 1860), ib., XII. 185. 



Leila (Yoldia) Evanxi, Meek and Haydeu (October, I860), ib., 429. 



Shell transversely elongate-subelliptical, depressed, rather compressed, 

 or a little gibbous in the central and umhonal regions; both extremities 

 narrowly rounded, the most prominent part of each being above the middle ; 

 basal margin nearly straight, and parallel to the cardinal border along the 

 middle, and rounding up gradually in front and behind ; dorsal margin nearly 

 straight, provided, behind the beaks, with a shallow groove in each valve ; 

 beaks depressed, small, and almost in contact, located a little in advance of 

 the middle of the shell; surface smooth, excepting very obscure traces of line 

 concentric stride. 



Length of largest specimen, 0.77 inch ; height, 0.36 inch ; convexity, 

 0.26 inch. 



Some internal casts of this species, obtained since it was first described 

 by us, show that the posterior muscular impression is very narrow-oval, and 

 placed with its longer diameter nearly horizontal. The anterior muscular 

 impression is larger than the other, of an oval form, and stands with its 

 longer diameter arranged nearly vertical. In some specimens, it appears to 

 have had an obscure ridge extending about half-way down, from above, near 

 the middle, so as to present a slightly bilobate aspect. These casts also show 

 that the pallia! line is provided with a rather deep, rounded sinus (see fig. 

 10c), as in the genus Yoldia, to which we have removed it. 



In some respects this species resembles the last, but it attains a much 

 larger size, is always more compressed, has less elevated beaks, and is propor- 

 tionally more broadly rounded posteriorly. It also wants the rather distinct 

 concentric stria? of that species, being almost perfectly smooth, excepting 

 very obscure traces of concentric lines. 



Among foreign species, one of its nearest representatives is Y. scapha 

 (r= Leda scapha, d'Orbigny, Palseont. Fr., Ill, pi. 801, fig. 2), from which 

 it differs, however, in having less elevated beaks, and in being much broader 

 behind the beaks, the posterior side of Y. scapha being subrostrate. 



Locality and position. — Moreau River, in the Fox Hills group ; and at 

 localities on the Yellowstone River with the last-described species: also, on 

 Milk River, apparently in the upper part of Fort Pierre group. 



