LAMELLIBRANCHIATA. 509 
Modiolopsis 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 Modiolopsis, 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, Chatfield, Minnesota. 
Mop1oLopsis OBSOLETA, 2. 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-fifth 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 and 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. Avisterella 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 wmbonata, Matheria rugosa and other species 
marking the upper part of the middle third of the Trenton shales near Cannon Falls, Minnesota. 
Moproxtopsts concava Ulrich. 
PLATE XXXVI, FIGS. 15, 16, 16a: 
Modiolopsis concava ULRicu, 1892. Nineteenth 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 much 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 
