132 PALEONTOLOGY. 



longing- to both of these ages occur there. As elsewhere explained, the 

 fossils on the lower half of this plate were figured together, because some 

 doubts at first existed in regard to the exact horizons of the beds from which 

 they came. Some of them are certainly Carboniferous; while others are 

 more like Jurassic forms. 



MYTILIDiE. 



Genus VOLSELLA, Scopoh. 



VOLSELLA SOALPRUM, var. ISONEMA. 



Plate 12, figs. 4, 4 a. 



Modiola scalimim, Sowerby (1821), Miti, Coiicb., Ill, 87, pi. 248, fig. 2. 

 Mytilus scalprum, Goldf. (1833), Petref. Germ., II, 174, tab. 130, fig. 9. 



Shell attaining a moderate size, extremely thin, transversely elongated, 

 or about twice and a half as long as high, gibbous along the umbonal slopes, 

 and rather distinctly arcuated ; hinge-line apparently nearly half the length 

 of the valves, passing gradually into the slope of the posterior dorsal curve ; 

 posterior margin curving obliquely backward and downward to the rather 

 narrowly-rounded posterior basal extremity ; pallial margin broadly sinuous 

 or arched, so as to be nearly parallel to the dorsal and posterior dorsal out- 

 line; anterior margin rounding up to the beaks, which are much depressed, 

 extremely oblique, very slightly projecting, somewhat compressed, and 

 placed nearly over the anterior end; umbonal slopes very prominently 

 rounded, so as to form an oblique ridge, extending from near the beaks to 

 the posterior basal extremity; above and behind this ridge, the surface is 

 slightly convex, while the flanks below it are more or less concave. Sur- 

 face ornamented with very fine and perfectly regular concentric lines. 



Length, 1.95 inches; height, 0.77 inch; convexity, about 0.60 inch. 



This shell agrees so very nearly in form and general appearance with 

 Modiola scalpriiin of Sowerby, and especially with a form referred by Gold- 

 fuss and others to that species (see Petref Germ., plate 130, fig. 9), that I 

 am left in doubt whether it is not a mere variety of the same. It agrees 

 less nearly with Sowerby's original figure, but his illustrations are not usually 

 so accurate as those published by Goldfuss ; while the figure referred to 

 in the work of the latter author is, I believe, generally regarded as repre- 



