410 THE PALEONTOLOGY OF MINNESOTA. 



[Lepticna cbarli>ttu>. 



the muscular area is also more elevated, and 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 affinities. Strophomena can be distinguished at once from 

 Rqfiesquina 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 D&lma,n=Producta rugosa Hisiuger=Conchites rhomboidalis 

 Wilckens. 



The following American species are of this genus: 



L. charlottcB W. and S., Trenton Shales. 

 L. tenuistriata Sowerby, Trenton to Hudson River. 

 L. unicostata Meek and Worthen, Hudson River. 

 L. rhomboidalis Wilckens, sp., Niagara to Waverly. 

 L. rhomboidalis, var. ventricosa Hall, Oriskany. 



Leptcena incrassata Hall, of the Chazy (Pal. N. Y., vol. i, p. 19), and Strophomena nitens Billings 

 of the Hudson River group (Pal. Fos., vol. i, p. 118) may also belong here. 



LEPT^ENA CHARLOTTE W. and S. 



PLATE XXXII, FIGS. 15. 



1892, April 1. Leptcena charlottes W. and S. American Geologist, vol. ix, p. 288. 

 1892, April 9. Strophomena halli SAKDESON. Bulletin of the Minnesota Academy of Natural 



Sciences, vol. iii, p. 334, pi. iv, figs. 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 



