410 THE PALEONTOLOGY OF MIN^TESOTA. 
[Leptaeaa charlottae. 
the muscular area is also more elevated, aud in the short, deeply impressed muscular 
area of the ventral valve. The surface corrugations are also developed among species 
of Rafinesquina and Strophomena. In the former, however, this character is never 
strongly marked, while the quadrangular outline and internal features will readily 
indicate their generic afl&nities. Strophomena can be distinguished at once from 
Bqfiesguina or Leptcena by the reversal of the relative convexity of the valves. 
For a detailed diagnosis of this genus, and for the reasons for restricting Leptcena, 
Strophomena, Rafinesquina and Plectambonites to the characters of their type species, 
• see Pal. N. Y., vol. viii, pp. 245, 276, 281. 
Type: Leptcena rugosa Dalman=Proc?wc^a rugosa Hisinger=Cowc^i7es rhomhoidalis 
Wilckens. 
The following American species are of this genus: 
L. charlottce W. and S., Trenton Shales. 
L. tenuistriata Sowerby, Trenton to Hudson River. 
L. unicostata Meek and Worthen, Hudson River. 
L. rhomhoidalis Wilckens, sp., Niagara to Waverly. 
L. rhomhoidalis, var. ventricosa Hall, Oriskany. 
Leptcena incrassata Hall, of tbe Ghazy (Pal. N. Y., vol. i, p. 19), and Strophomena nitens Billint^s 
of the Hudson River group (Pal. Fos., vol. i, p. 118) may also belong here. 
Lept.*;na CHARLOTT.fi W. and S. 
PLATE XXXIL FIGS. 1-5. 
1892, April 1. Leptcena charlottce W. and S. American Geologist, vol. ix, p. 288. 
1892, April 9. Strophomena halli Sardeson. Bulletin of the Minnesota Academy of Natural 
Sciences, vol. iii, p. 334, pi. it, flgs. 36-38. 
Shell small, transversely semioval, plano-convex, geniculate, with the sides 
slightly convex and converging to the broadly rounded front, or drawn out tongue- 
shaped; hinge-line as long as, or somewhat shorter, than the greatest width of the 
shell. Surface marked by fine, closely crowded, alternating striae, as in Rafinesquina 
alternata, crossed by exceedingly delicate concentric lines and over the central flat 
disc of each valve by more or less continuous zigzag undulations or wrinkles. 
Ventral valve depressed-convex over the greater portion of the shell and more 
or less suddenly bent downward or geniculated along the margin, especially anteri- 
orly. Cardinal area wide, broadly triangular, with a convex deltidium, wider than 
long, apically perforated by a rather large pedicle opening, posteriorly excavated 
and completely occupied by the chilidium of the other valve. Crenulated hinge 
teeth prominent and supported by short dental plates, which are attached to the 
elevated outer margin of the small, transversely oval muscular area. Within this 
area, in the center of the mesial thickening, are placed the short and narrow 
adductors, surrounded by the large diductors, and outside these, at the base of the 
