LAMELLIBRANCHIATA. 507 



Modiolopsis nana.l 



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 Mi arguta line traces by very gradual degrees into M. concentrita H. and W., 

 which is a Modiolopsis in every respect. 



M. nana, of the Galena shales, has stronger concentric striae, 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, angustatd 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,' Chatfleld and 

 Fountain, Minnesota. 



Mus. Beg. No. 8350. 



Modiolopsis nana, n. 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 umljonal 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 continu.e 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. 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 Gannon Falls, Minnesota. 



