402 THE PALEONTOLOGY OF MINNNSOTA. 



[Rafiaesquma minnesotensis 



scars], between the anterior ends of which rises a short mesial ridge of about the 

 same size and length, with faint linear ridges parallel with it on each side, which 

 extend a little further forward than the mesial ridge. The mesial ridge first gives 

 place to a flat, unmarked interval, when it again rises more conspicuously, but nar- 

 rower and sharper, extending nearly to the sinus separating the lobes of the outer 

 muscular scar. The cardinal area of the convex valve slopes from the hinge-line 

 obliquely backward, instead of being in plane with the lateral edges, thus differing 

 from S. alternata. From three to five short undulations of the shell transverse to 

 to the cardinal line, are seen often between the umbo and the cardinal angles, the 

 heavier ones being near the cardinal angles. The cardinal process is bifid and 

 prominent, the two parts being short, smooth, dentate protuberances that stand 

 prominently exposed about parallel with the plane of the cardinal area. 



"The interior of the entering [dorsal] valve is very different from that of the 

 entering valve of S. alternata. The general visceral disc is nearly flat, surrounded 

 by a suddenly flexed margin, inside of which is a shallow impressed broad line, most 

 evident round the front; inside the cardinal angles are a few scattered, radiately- 

 interrupted, short ridges or elevations [genital markings], but these do not prevail 

 along the side nor in front, the surface there being smooth or finely granulated 

 instead; in the center of the valve are five smooth, abrupt, digitately-spreading 

 ridges, the middle one of which is a little larger and longer than the others; these 

 rise more abruptly at their anterior extremities than behind, but none of them 

 reach the bpak, or even the umbonal region, though the exterior pair of lateral ones 

 are placed further back than the others, converging at an angle of about 70 [and 

 often pass through the large pair of adductor scars]. Socket [crural] ridges very 

 short and widely divergent; behind them are small, doubly-grooved sockets." The 

 beak of the ventral valve is often perforated by a minute, circular, pedicle opening. 



R. incrassata Hall* seems to be a closely allied species, of which a few examples 

 from the typical locality have been examined, but they are too poor for detailed 

 comparison. These specimens and Prof. Hall's figures of the species are constant in 

 size and always smaller than E. minnesotensis. Billings,! however, found R. incrassata 

 ,'in the Chazy limestone at the Mingan islands, and in the Black River limestone at 

 the Fourth Chute of the Bonnechere, Canada. Specimens from the latter locality 

 agree precisely with those sent me from Tennessee by Prof. Safford." Of the Ten- 

 nessee specimens referred to by Billings, the writers possess a complete series, and 

 find them to be identical with R. minnesotensis Winchell. It is probable that R. 

 minnesotensis is only a larger development of the eastern R. incrassata. 



Lcptcena iwrassata Hall. Pal. N. Y., vol. 1. p, 19, pi. iv bis, flgs. 2a-2d. 

 tCanadian Nat, and Geol., vol. Iv, p. 443, 1859, 



