506 THE PALEONTOLOGY OF MINNESOTA. 
{(Modiolopsis oweni 
This species ought, perhaps, to go with Actinomya rather than Modiolopsis, but 
as I have so far seen only the exterior of the shell, and therefore know nothing of 
the internal characters, it seemed best to refer it to Modiolopsis provisionally, because 
of a general resemblance to M. similis. I wish to say further, that I would not be 
surprised if the shell proved to have the hinge of a Cyrtodonta, several species of 
which it resembles quite as much as it does Modiolopsis. | 
Formation and locality.x—Near the base of the Trenton formation, Murfreesboro, Tennessee. 
MopIoLoPsis OWENI, 2. Sp. 
PLATE XLII. FIGS. 15 and 16. 
This species is founded upon a single and not very well preserved cast of the 
interior. It seems to belong to Modiolopsis and very near M. similis, with which 
species it should be compared. As far as can be seen its valves were a little more 
convex, the mesial sulcus narrower, the anterior part of the shell somewhat inflated 
and the posterior part comparatively narrower. 
Formation and locality.—Galena shales, about five miles south of Cannon Falls, Minnesota. 
MopIoLoPsIs ARGUTA, 2. Sp. 
PLATE XXXVI, FIGS. 3-6. 
Shell small, ventricose, elongate, highest posteriorly, the length twice the 
ereatest hight, and three times the hight at the beaks. Cardinal margin straight; 
anterior end unusually long, sharply rounded at the extremity of the hinge beneath 
which it slopes backward gradually curving into the straight ventral margin; poster- 
ior end strongly convex and most prominent in the lower half, above curving more 
gently and very gradually into the dorsal edge. Beaks a little more than one-sixth 
of the length from the anterior extremity, moderately prominent and incurved, com- 
pressed; mesial impression scarcely more than a mere flattening of the sides of the 
shell; umbonal ridge rather sharply rounded. Point of greatest convexity of valves 
very near the center. Surface with concentric lines, sharp, subequal and thread- 
like on the cardinal slopes, here with about ten in 5 mm. at their strongest parts, 
becoming faint before they pass over the umbonal ridge in their course to the 
anterior end where they are again somewhat thread-like. In good casts of the 
interior the anterior adductor scars are large, prominent, and marked on their inner 
halves with transverse lines. The surface markings do not show through the shell 
so as to mark the casts. Hinge thin, apparently edentulous. An average specimen 
is 24 mm. long, the largest seen about 31 mm. 
This is one of a number of closely related species ranging from the lower Trenton 
to the middle beds of the Cincinnati*group. They are all elongate, especially so for 
