LAMELLIBRANCHIATA. 507 
Modiolopsis nana.) 
Modiolopsis, anterior to the beaks. Their general expression, therefore, is decidedly 
like Orthodesma, of which some member of this line is believed to have been the 
ancestor. In Orthodesma the valves gape slightly at the ends, which is not the case 
in these shells. In that genus again the point of greatest thickness is more or less 
behind the center, while in all the species referred by me to Modiolopsis this point is 
central or anterior to the center. Furthermore, as stated under the generic descrip- 
tion, the VW. arguta line traces by very gradual degrees into M. concentrica H. and W., 
which is a Modiolopsis in every respect. 
M. nana, of the Galena shales, has stronger concentric striz, and these extend 
further forward and are visible on the internal cast, is scarcely so convex, with a 
deeper mesial depression and more obtuse umbonal ridge, and more rounded and 
shorter anterior end; M. mytiloides Hall, is without the even thread-like lines on the 
cardinal slope; and M, angustata Ulrich, of the Cincinnati rocks, has a more truncate 
posterior margin, more uniformly rounded anterior end, and more nearly parallel 
dorsal and ventral margins. 
Formation and locality.—Middle third of the Trenton shales, Minneapolis, St. Paul, Chatfield and 
Fountain, Minnesota. 
Mus. Reg. No. 8350. 
MopIoLopsis NANA, %. Sp. 
PLATE XXXVI, FIG. 7. 
This small species is closely related to M. arguta.- The differences are as follows: 
The valves are not quite as convex, the umbonal ridge is less sharply rounded, the 
mesial depression a trifle deeper, and the anterior end a little shorter and more 
uniformly rounded. The most striking peculiarity, however, is found in the con- 
centric lines which show very distinctly on casts of the interior, are coarser (eight 
in 5 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. 7 
In M. mytiloides Hall, as identified in Minnesota, the surface of the casts is very 
obscurely marked with concentric lines, and the posterior extremity of the hinge 
line subangular. 
Only two specimens have been seen. Of one the length is 19 mm., the posterior 
hight 9.3 mm., the anterior hight 7.2 mm., the thickness 6 mm. Of the other these 
dimensions are respectively 16, 8, 6 and 5 mm. 
Formation and locality —Galena shales, near Cannon Falls, Minnesota. 
