42 Trans. Acad. Sci. of St. Louis. 
shell is slightly convex and passes downward at an angle 
of seventy-five degrees with the hinge line; lower margin 
strongly and evenly convex; posterior margin com- 
pressed, slightly convex, meeting the hinge line at an 
angle of one hundred twenty degrees. Surface of the 
shell marked with fine concentric lines, indistinct in casts. 
M. subrhomboideus differs from M. gibbous, the near- 
est allied species, in having a tooth at the posterior end 
of the hinge plate, in the larger angle that the anterior 
margin of the shell forms with the hinge line, and in its 
rhomboidal shape. 
This species is based on five fairly well preserved 
specimens. Number 11518 of the invertebrate paleon- 
tological collection of Walker Museum. 
MODIOLOPSIS EXPANSA, N. Sp. 
Plo. f. 8-6, 
Shell small, length eight millimeters, greatest height 
five millimeters, about twice as high at the posterior end 
as at the beaks. Hinge plate long, straight, apparently 
edentulous. Anterior end of shell subangular, project- 
ing fartherest at the ventral margin. Ventral margin 
convex anteriorly and posteriorly, concave in the middle, 
posterior margin convex, straightening slightly above 
where it passes backward to meet the hinge line. A 
deep sulcus passes downward and backward from the tip 
of the beaks meeting the ventral margin at about the mid- 
dle of the shell. Surface of shell with concentric lines, 
not well preserved in the types which are casts. Anterior 
adductor sears large round and deep, occupying most of 
the shell anterior to the beaks. 
M. expansa differs from M. nana, the nearest allied 
species in being much higher posteriorly, having a deeper 
suleus, and the adductor scars being smaller and deeper. 
The species is based on five fairly well preserved casts. 
They are number 11521 of the invertebrate paleontolog- 
ical collection of Walker Museum. 
