402 THE PALEONTOLOGY OF MINNNSOTA. 
{Rafinesquina 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 Hexed 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 beak, 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 Ff. 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 Bonnechére, 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 FR. minnesotensis Winchell. It is probable that R. 
minnesotensis is only a larger development of the eastern R. incrassata. 
* Leptceena incrassata Hall. Pal. N. Y., vol. i, p, 19, pl. 1v bis, figs, 2a-2d. 
+Canadian Nat. and Geol., vol. iv, p. 443, 1859. 
