690 



THE PALEONTOLOGY OF MINNESOTA. 



IBythooyprls? robusta. 



ByTHOCYPRIS (?) ROBUSTA, U. Sp. 



rig. 52.— a, a left valve, veith somewhat blunter ends than usual, showing position of subcentral 

 spot; 6, right side of entire carapace; c and d, ventral and posterior outline views of same; lower Trenton 

 ("Lower Blue limestone"), Dixon, Illinois. 



Size.— Length 2.5 mm.; hight 1.05 mm.; thickness 0.87 mm. 



Carapace elongate, subelliptical or obscurely triangular, the outline convex 

 dorsally and nearly straight or gently arcuate ventrally; anterior end strongly 

 rounded and somewhat higher (blunter) than the posterior. In a ventral view the 

 outline is wedge-shaped, the higher or anterior end being much more attenuate 

 than the other. Valves rather strongly convex, unequal, the left overlapping the 

 right all around except at the posterior extremity; dorsal edge of left valve some- 

 what thickened; posterior extremity of right (smaller) valve subacute; surface 

 smooth; each valve occasionally showing a small discolored spot a. short distance in 

 front of the center. 



The affinities of this fine .species are doubtful. It is not a true Bythocypris, nor 

 is it any more like either Macrocypris or Pontocypris. Perhaps it should be placed 

 under Bairdia since it resembles certain species that have been referred to that 

 genus by Jones and HoU. Yet, after a careful comparison with numerous Devonian 

 and Carboniferous species of Bairdia, I have come to doubt the propriety of recog- 

 nizing that genus in any of the known Lower Silurian species. The acuminate 

 posterior extremity of the right valve reminds as much of the new genus Krausella, 

 and it is an alliance with this genus that I would favor more than with Bairdia. 



Specifically, B. ? robusta will be distinguished at once from all known Lower 

 Silurian Gypridce by its large size. 



Formation and looality.— Lower Trenton limestone, Dixon, Illinois, where it was found abundantly 

 in association with Krausella inoeqwalis and Schmidtella crassimarginata. 



