510 : THE PALEONTOLOGY OF MINNESOTA. 
{[Modiolopsis concentrica 
width of the mesial sulcus and the rapid descent of the surface included in it. 
Umbonal ridge slight, cardinal slope, convex. Ina dorsal view the anterior half of 
the shell appears compressed, yet the point of greatest thickness is very near the 
middle of the length. Surface marked with simple concentric lines of growth. 
Hinge plate very thin, without teeth or appreciable thickening under the beak. 
Muscular scars not observed. 
This peculiar species, which is decidedly mytiloid in appearance and probably 
not a true Modiolopsis, is distinguished at once from all known Lower Silurian 
Lamellibranchiata, except M. arcuata Hall, by its strongly arcuate form, Hall’s 
species is represented as larger and with a straight instead of convex back. 
Formation and locality —Same as the preceding. 
Moptonopsis concentrica Hall and Whitfield. 
* PLATE XXXVII, FIGS. 15 and 16. 
Modiolopsis concentrica HALL and WHITFIELD, 1875. Pal. Ohio, vol. ii, p. 86. 
Shell rather exceeding medium size, elongate ovate, highest in the posterior 
half. Hinge line arcuate, gently declining toward the extremity and rounding gradu- 
ally into the oblique posterior margin, the same curve continuing to the lower third 
when it is sharpened in turning forward into the basal margin. The latter is gently 
convex in the posterior half and anterior third, the part between being very slightly 
concave. Anterior end very short, narrowly rounded. Beaks small, compressed, 
projecting very little above the hinge. Surface of valves moderately convex, most 
prominent a little in front of and above the middle; this point is on the umbonal 
ridge, which is low, broadly rounded, and not a conspicuous feature. Mesial sulcus 
shallow, forming an undefined depression across the valves from the beak to the 
middle third of the basal margin. Surface marked on the cardinal slope and poste- 
rior end by regular, even, concentric furrows, four to six of them in 5 mm. in their 
strongest parts. These furrows are most distinct along a line following the middle 
of the cardinal slope; in crossing the umbonal ridge they become suddenly obsolete, 
existing On the sides, basal portion, and anterior end only as fine irregular strize of 
growth. 
In casts of the interior the concentric furrows are distinctly visible on the 
posterior half of the cardinal slope. The mesial sulcus is much deeper and rather 
sharply defined on the posterior side by a strongly convex ridge extending obliquely 
across the cast from a point a short distance behind the beaks toward the basal 
margin, which, if the ridge did not become obsolete before reaching it, would he 
intersected at a point about three-fifths of the length of the shell behind the anterior 
extremity. In front of this ridge the surface is impressed and flattened to the 
