LAMELLIBRANCHIATA. 505 



Modiolopsis oonslmills.] 



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 

 £he shell from the beak and, expanding, causes the straightening of the ventral rqar- 

 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 AcUnomya 

 and true Modiolopsis. This is seen in theAhin shell and consequent indistinctness . 

 of the anterior adductor impression, in the full and prominent umbones and in 

 the convex rather than straight or concave baisal line. At first I was inclined to 

 put the species into the new genus, but later comparisons have shown that Adino- 

 mya was at that time already well established and that M. similis belongs to the line 

 which finally produced M. modiolaris. Then the. comparatively strong mesial depres- 

 sion indicates Jlfoc^w/ops^s and not AcUnomya. 



Compared with Minnesota Trenton species, all the others referred to Modiolopsis 

 are narrower posteriorly. The AcUnomya 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 G. subovata Ulrich. 



Formation and loaliiy.— Middle third of the Trenton shales at Minneapolis, Minnesota. 



Modiolopsis (?) consimilis, n. sp. 



PLATE XLir, PIGS. 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 

 diflerent, the posterior hight being relatively somewhat less than in the preced- 

 ing species. 



