MKU.II.l; \N< IIIA 1 A 

 mim.l 



i to the Leaks. Tlu-ir L'eneral expression, therefore, i- decidedly 

 like n,-tli,l,sniii. of which some niemi.er of this lin<> is believed to have Keen the 

 ancestor, in ,///,.,,, the \ahes gape slightly at the ends, wliirli is ,,,,t the case 

 in these -hells. In that gemis again the |.., m t of greatest thickness i- more or less 

 behind the center, while in all the referred by me to .l/-.,//W.//.x/x this point i> 



central or anterior to t h.> r,.i)t.-r. Kurt h.-rinon', iis stated under the generic .!-. rip 

 tion. tin- \l ,-.i,,la line tract's -nidual .It'Kivcs into M mncrntrini II. ami \V.. 



\\hii-li is a M"ilii>l<>i>sis in every respec-t . 



M it'iixi. of the (ialena shale*, has stronger concent n.- -trui', and these extend 

 further forward an. I are vi.-ilile on the internal cast, is scarcely so convex, with a 

 deeper ine-ial depression and more obtuse umbonal ridge, and more rounded and 

 -horter anterior end; M. ii/iili,l^ Mall, is without the even thread-like lines on the 

 cardinal >lope; and M. <in>jn*l,it<i I Irich, of tlu> Cincinnati rocks, has a more truncate 

 posterior margin, more uniformly rounded anterior end, and more nearly parallel 

 dorsal and ventral margins. 



tation and locality. Middle thlnl ..( ih. Tr.-ni..ii shale*, Minneapolis St. Paul, Chatflcld and 

 .tain. Mlnnt-soU. 



Hit*. Rty. No. 8360. 



MODIOLOPSIS NANA, H. ftp. 

 I'LATF. XXXVI. KIQ. 7. 



This small species is closely related to M. argtita. The differences are as follows: 

 The valves are not quite as convex, the umbonal ridge is less sharply rounded, the 

 mesial depression a trifle deei>er, and the anterior end a little shorter and more 



uniformly rounded. The most -triking peculiarity, however, is found in the con- 

 ric lines which show very distinctly on casts of the interior, are coarser (eight 



in ") mm.), more regular and continue of the same strength over the cardinal slope, 



umbonal ridge and forward into the mesial depression, near the center of which 



they are lost 



In M. tnyti/i,:./,. Hail, as identified in Minnesota, the surface of the casts is very 

 iirely marked with concentric lines, and the posterior extremity of the hinge 



line suliangiilar. 



Only two -penmen- have been seen. Of one the length is I'.i mm .. the posterior 



higlit '.!.:{ mm., the anterior bight 7/J mm., the thickness 6 mm. Of the other these 



dimension* are ,ely If,. S, 6 and ". mm. 



Formation and loealilf.Gnlrn* "hales, nnar Cannon Falls, Minnesota. 



