INVERTEBRATE PALAEONTOLOGY. 73 



Volsella Galpiniana, E. & s. (sp.). 



Plate 2H, figs. 7, «, b. 



Mytilua Galplnianus, Evans ami Shuuiard ( 1834 i. Proceed. Acad. N. s. Philad., VII, 104. — Meek and Hay- 

 den (1860), ib., xii, 427. 



Shell transversely arcuate-subovate, gibbous along the umbonal slopes, 

 and cuneate posteriorly; surface marked only with fine concentric stria 1 ; 

 anterior end very short ; beaks very oblique, placed over the anterior margin, 

 and somewhat compressed; hinge-margin a little straightened for near 

 half the length of the valves, and rounding off imperceptibly into the poste- 

 rior margin, which curves obliquely to the narrowly-rounded posterior basal 

 extremity; basal margin strongly arcuate along the middle, near which the 

 Hanks arc compressed and contracted below the umbonal ridge; anterior 

 muscular impression distinct. 



Length, 1.45 inches; height, 0.71 inch; convexity, 0.46 inch. 



The cardinal border of this species is somewhat arched, and less than 

 half the length of the shell. It forms an angle of about 40° with the um- 

 bonal axis, and at its posterior extremity passes imperceptibly into the posterior- 

 dorsal edge, which is carinate, and descends with a graceful curve to the 

 narrowly-rounded lower part of the anal extremity. The anterior side of the 

 shell is narrower than the other, and its border below the beaks projects but 

 slightly beyond them. The most convex portion of the valves forms a kind 

 of obscure ridge or prominence, extending with a slight curve from the beaks 

 obliquely backward toward the lower part of the posterior border, while in 

 front of, and below, this ridge, the valves are more compressed. Our speci- 

 mens are all internal casts, showing the very narrow-oval anterior muscular 

 impression, and pallial line, quite distinctly, but preserving only obscure traces 

 of the larger posterior muscular scar, from which it may be inferred that the 

 latter was generally very faintly marked. 



This species will be easily distinguished from the preceding by its more 

 arcuate form, broader posterior extremity, and more compressed beaks. It 

 resembles more nearly several other described forms in rocks of various ages, 

 among which may be mentioned the Oolitic species M. Leckenbii of Morris and 

 Lycett; from which, however, it differs in being less strongly arcuate, and not 

 near so distinctly angular along the middle of the valves. 



Locality and position. — Long Lake, Fox Hills, and on Moreau River; in 

 the Fox Hills group, or formation No. 5 of the Upper Missouri Cretaceous 

 series. 



10 H 



