448 THE PALEONTOLOGY OF MINNESOTA. 
LOrthis (Dalmanella) subaequata. 
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, gihhosa 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, 'Vihile the circular forms, with fine and equal striae, 
are placed in the variety circularis. 
The types of Conrad's 0. perveta and 0. suhceqiiata, 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 diflerences 
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 l)y Conrad, as shown by the 
specific name. They differ from 0. subcequata only in having coarser stria?, 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. — Rare 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." ^Billings, Can. Nat. Geol., vol. iv, p^434.) 
Collectors.— mss C. S. Seymour, H. V. Winchell, W. H. Scofleld, W. H. Shelton, C. L. Herrick, E. O. 
Ulrich and the writers. 
Mus. Beg. Nos. 321, 346, 374, 644, 648, 707, 720, 737, 739, 766, 789, 794, 3513, 3519, 4032, 4056, 4939, 
4943, 4975, 4976, 5058, 5093, 5581, 5671, 6775, 6792, 6801, 6803, 7915, 7959-7968. 
