448 THE PALEONTOLOGY OF MINNESOTA. 



[Ortlils (Diilniiiiielh 



valve. The delthyrial cavity is more or less thickened, excavated anteriorly and 

 produced centrally into a low, but distinct, median septum, which terminates at 

 about the mid-length of the valve and separates the two paris of adductor scars, the 

 posterior pair being slightly smaller. When the scars are more divergent than 

 usual the septum is thickened laterally and fills the space left between the anterior 

 pair. Surface in front of the scars marked with a few faint, short, radiating lines 

 of the vascular system. 



This widely distributed species is nearly always prolific in individuals at most 

 localities and varies considerably in outline, number of striae and in the mesial fold 

 and sinus. The writers have several hundred examples, together with the varieties 

 conradi, perveta, gibbosa and circularis, the greater number of which were collected in 

 Minnesota, though specimens were also obtained from Canada, Kentucky, Tennessee, 

 Illinois, Iowa and Wisconsin. In southern localities the circular form prevails, while 

 in the northwest the species is commonly wider than long. In the Black River 

 limestone of eastern Canada, a well developed fold and sinus, combined with a more 

 or less short hinge-line, is the local change. These varieties also occur in the Trenton 

 shales between the cities of Minneapolis and St. Paul, and are often not sufficiently 

 constant to permit the positive separation of a large lot of individuals. This, how- 

 ever, is to be expected in any prolific and plastic species. The specimens with coarse 

 striae are separated as var. perveta; those with a more or less profound and angulated 

 sinus are referred to var. gibbosa, while the circular forms, with fine and equal striae, 

 are placed in the variety circularis. 



The types of Conrad's 0. perveta and 0. subcequata, now preserved in the American 

 Museum of Natural History in New York city, have been compared with similar speci- 

 mens from Mineral Point, Wisconsin, the original locality, and no specific differences 

 between them have been made out. In Pal. New York, vol. i, 0. perveta is figured 

 as a small species, but in the Geol. of Wisconsin, Prof. Hall states that it attains a 

 width of one inch. Numerous specimens, however, occur at Mineral Point which 

 agree with the above illustrations of Conrad's type of 0. perveta. These appear to 

 be adult individuals and were probably so regarded by Conrad, as shown by the 

 specific name. They differ from 0. subcequata only in having coarser striae, and the 

 name perveta is retained for them as a variety. 0. minneapolis Winchell proves to be 

 identical with 0. subcequata, while 0. conradi Winchell, was applied to a small but 

 mature form of the same species occurring in abundance in the Trenton limestone. 



Formation and locality. Bare in the Trenton limestone at Minneapolis and Rochester, Minnesota. 

 Common in the Trenton shales at Minneapolis, St. Paul, Cannon Falls, Lanesboro, St. Charles, Eyota, 

 Fountain, Fremont, Chatfleld, Preston and near Caledonia, Minnesota; Decorah and McGregor, Iowa. 

 Near the base of the Upper Buff limestone at Mineral Point, Wisconsin; Auburn, Lincoln county, Missouri. 

 "Of O. subcequata I have seen only a single specimen. It was found in an old quarry two miles north of 

 Montreal." vBillings, Can. Nat. Geol., vol. iv. p. 434.) 



Collectors. Miss C. S. Seymour, H. V. Winchell, W. H. Scofleld, W. H. Shelton, C. L. Herrick, E. O. 

 Ulrich and the writers. 



Mas. Reg. Nos. 321, 346, 374, 644, 648, 707, 720, 737, 739, 766, 789, 794, 3513, 3519, 4032, 4056, 4939, 

 494H, 4975, 4976, 5058, 5093, 5581, 5671, 6775, 6792, 6801, 6803, 7915, 7959-7968. 



