132 PAL.T: ONTOLOGY. 



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

 fossils on the lower half of this phite 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 Hke Jurassic forms. 



MYTILIDyE. 



Genus VULSELLA, Scopoh. 



VOLSELLA SCJALPKUM, var. ISONEMA. 



Plate 12, figs. 4, 4 a. 



Modiola scalprum, Sowerby (1821), Min. Concb., Ill, 87, i»l. 248, fig. 2. 

 Mytilns scalpnm, 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, Avhich are much depressed, 

 extremely oblique, very slightly projecting, somewliat 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 Avith very fine and perfectly regular concentric lines. 



Length, 1.95 inches; height, 0.77 inch; convexity, about O.GO inch. 



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

 Modiola scalprjm 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 wliether 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 figm-e referred to 

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



