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. suhrhomhoideus 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, U. Sp. 



PL vii. f. 5-6. 



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. A^entral 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 scars 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 

 sulcus, 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. 



