504 THE PALEONTOLOGY OF MINNESOTA. 



[Modiolopsls 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, M. plana Hall, M. alata TJlrich 

 and perhaps M. truncata Hall, belong to Eurymya; M. oviformis Ulrich, to Modiolodon; 

 M. subelliptica Ulrich, to Allodesma; M. cincinnatiensis Hall and l^\i\i^e\di,M. pulchella 

 Ulrich, M. cancellata Walcott, M. oblonga Ulrich, M. pholadiformis Hall, and M. superba 

 Hall to Adinomya; 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 oi Goni- 

 ophora, Phillips. Of Upper Silurian species M. recta Hall, from the Niagara of Wis- 

 consin, is du'Matheria, while the M. dicteus of the same author and locality, and M. 

 primigenia Colirad, sp., of the Medina, have slender cardinal and posterior lateral 

 teeth of the Gyrtodonta type. 



Modiolopsis similis TJlrich. 



/ PLATE XXXVI, FIGS. 1 and 2; PLATE XLII, FIG. 19. 



]892. Modiolopsis similis Ulkich. Nineteenth Ann. Keport, 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 



