504 THE PALEONTOLOGY OF MINNESOTA. 
[Modiolopsis similis. 
its interior, in a casual glance, would pass, there being the same large and deeply- 
impressed anterior adductor scar, and nearly every feature with which those conver- 
sant with species of Modiolopsis are familiar. The exception is in the hinge, which 
is found to have a slightly oblique fold or tooth over the muscular scar in the left 
valve and a corresponding groove in the right. In true Modiolopsis this tooth is 
wanting, or rather, it is but little developed, since an obscure thickening of the hinge 
plate between the muscular impression and the beak is noticeable in many species 
of Modiolopsis. Another feature is observed in Modiomorpha concentrica that may be 
of importance. Namely, the hinge plates posterior to the beaks are wider than in 
any Modiolopsis known. They extend inwardly and at the same time diverge, probably 
for the reception of a strong internal ligament, the removal of the thin plate leaving 
a sharp slit a little within the cardinal edge of casts of the interior. The value 
of the character is to be tested only by its persistence in other species referred to 
Modiomorpha. It is a matter worthy of being looked into, for it must be admitted 
that another difference between Modiolopsis and Modiomorpha, besides the only one 
now recognizable, is, to say the least, desirable. : 
Of the numerous species which have been placed in this genus many proved 
distinct when subjected to critical study. Others look doubtful, but must remain 
here for want of material to determine their relations. Of those to be removed some 
fall under the new genera about to be proposed. Thus, /. plana Hall, M. alata Ulrich 
and_perhaps MtruncataHall, belong to Hurymya; M. oviformis Ulrich, to Modiolodon; 
M. subelliptica Ulrich, to Allodesma; M. cincinnatiensis Hall and Whitfield, M. pulchella 
Ulrich, M. cancellata Walcott, M. oblonga Ulrich, M. pholadiformis Hall, and M. superba 
Hall to Actinomya; M. gesneri Billings and M. trentonensis Hall, to Endodesma. M. 
nasuta Conrad, sp., and M. subnasuta Meek and Worthen, belong to Orthodesma, Hall 
and Whitfield, and M. carinata Hall, possesses all the essential characters of Goni- 
ophora, Phillips. Of Upper Silurian species MW. recta Hall, from the Niagara of Wis- 
consin, is a Matheria, while the M. dicteus of the same author and locality, and M. 
primigenia Conrad, sp., of the Medina, have slender cardinal and posterior lateral 
teeth of the Cyrtodonta type. 
Moprovorsis sruitis Ulrich. 
PLATE XXXVI, FIGS. 1 and 2; PLATE XLII, FIG. 19. 
1892. Modiolopsis similis ULR1cu. Nineteenth Ann. Report, Geol. Nat. Hist. Sur, Minn., p. 225. 
Shell of medium size, obliquely elongate ovate, highest in the posterior half, 
contracted at the beaks to between one-half and three-fifths of the greatest hight. 
Hinge line nearly straight, about half as long as the shell posterior to the beaks. 
Anterior end small, neatly rounded; ventral margin gently convex, nearly straight 
