LAMELLIBRANCHIATA. 505 
Modiolopsis consimilis.] 
in the middle; posterior end broadly rounded, slightly produced and more strongly 
convex in the lower half, the upper more gently curved and sometimes forming an 
obtusely angular junction with the hinge line. Beaks about one-seventh of the entire 
length of shell behind the anterior extremity, rather small, incurved, projecting 
moderately above the hinge; umbones compressed in the cast, a little less so in the 
shell. Surface moderately convex, most prominent along the umbonal ridge, the 
latter a little stronger than usual for species of this genus. Cardinal slope concave. 
A broad and comparatively well-defined mesial depression extends obliquely across 
the shell from the beak and, expanding, causes the straightening of the ventral mar- 
gin. Excepting in this part the shell is very thin, and the anterior muscular scar, 
which is comparatively of small size, is scarcely distinguishable in casts. Surface 
rather obscurely marked with numerous fine concentric lines and a few stronger 
varices of growth. 
As might be expected, this early species exhibits features intermediate between 
those marking the group of forms which I now propose to distinguish as Actinomya 
and true Modiolopsis. This is seen in the thin shell and consequent indistinctness 
of the anterior adductor impression, in the full and prominent umbones and in 
the convex rather than straight or concave basal line. At first I was inclined to 
put the species into the new genus, but later comparisons have shown that Actino- 
mya was at that time already well established and that J. similis belongs to the line 
which finally produced M. modiolaris. Then the comparatively strong mesial depres- 
sion indicates Modiolopsis and not Actinomya. 
Compared with Minnesota Trenton species, all the others referred to Modiolopsis 
are narrower posteriorly. The Actinomya superba Hall, sp., has a larger anterior 
end, the postero-basal margin more produced, and the umbones larger. The 
undescribed Kentucky species referred to in the original description proves to be 
a Cyrtodonta closely related to C. subovata Ulrich. 
Formation and loality.—Middle third of the Trenton shales at Minneapolis, Minnesota. 
Mopiotorsis (?) CONSIMILIS, 7. sp. 
PLATE XLII, FIGS. 17 and 18. 
This shell is so much like M. similis that at first I believed it might belong 
to the same species. Carefully compared, however, it proved to differ in several 
characters that are more important than striking. The umbones are larger 
and very little compressed, and the mesial sulcus, which is a well marked 
feature in that species, is scarcely distinguishable. The outline also is a little 
different, the posterior hight being relatively somewhat less than in the preced- 
ing species. “ 
