LAMBLLIBRANCHIATA. 509 



Modlolopsis obsoleta.] 



nearest M. arguta, with which it is also associated. It is, however, readily distin- 

 ugished by its surface markings, which are not visible on the casts of that species, 

 and by its less oblique anterior and posterior ends and more nearly parallel ventral 

 and dorsal margins. In M. faba Hall, which is probably not a true Modlolopsis, the 

 mesial depression is much more distinct. . M. nana is wider and more oblique pos- 

 teriorly, and has more regular surface markings. 



Formation and locality. — Middle third of the Trenton shales, Chatfleld, Minnesota. 



MODIOLOPSIS OBSOLETA, '«. Sp. 

 PLATE XXXVI, FIGS. 11 and 12. 



Shell small, elongate ovate, the length twice the greatest or posterior hight. 

 Valves thickest a little above the center, rather uniformly convex, the umbonal 

 ridge and mesial depression being both nearly obsolete. Beaks small, between one- 

 fourth and one-flfth of the entire length from the anterior extremity. Dorsal margin 

 gently arcuate, anterior end narrowly but almost uniformly rounded, ventral edge 

 straight, posterior end slightly oblique, rather broadly rounded, most prominent a 

 little beneath the center, above which it curves forward gradually into the hinge 

 line. Surface with very fine concentric lines; these are equal an4 strongest near the 

 posterior cardinal border. Hinge very thin, edentulous. Muscular scars not observed. 



Length 13.3 mm., posterior hight 6.6 mm., anterior hight 5 mm., thickness (left 

 valve only) about 2.5 mm. 



Considerably like, and probably a near relative of M. arguta, but differs in the 

 more uniform convexity of its surface, obsolete umbonal ridge and less oblique 

 anterior margin. The posterior end also is comparatively narrower' and the shell 

 smaller. Aristerella nitidula is associated but cannot be confounded, since it is a 

 smooth shell, with unequal valves, and much wider posteriorly. 



Formation and locality. — Associated with Plethocardia umbonata, Matheria rugosa and other species 

 marliing the upper part of the middle third of the Trenton shales near Cannon Falls, Minnesota. 



MoDioLOPSis OONCAVA TJlHch. 



PLATE XXiVI, FIGS. 15, 16, 16a. 



Modiolopsis concava Ulrich, 1892. Niueteenth Ann. Rep. Geoi. Nat. Hist. Sur. Minn., p. 227. 



Shell very small, elongate, the greatest hight a little less than the length, 

 arcuate, the posterior end mu(?h the widest and broadly rounded, the anterior end 

 exceedingly short, narrow and contracted beneath the beaks; the latter are small, 

 compressed, and project but little above the hinge. Hight of posterior third about 

 two and one-half times as great as at the beaks. Dorsum gently arcuate ; anterior 

 two-thirds of ventral margin strongly concave, a fact due in a great measure to the 



