450 THE PALEONTOLOGY OF MINNESOTA. ¢ 
[Variety perveta. 
divaricator process was seen in any silicified specimens observed. In O. subaquata, 
var. conradi, these parts are comparatively as strongly developed as in any of the 
larger forms of this species occurring in the shales above. 
These small specimens are regarded as mature but dwarfed individuals of O. 
subequata, and since they hold a constant horizon over a great area, the varietal 
name conradi will serve to distinguish them from the other varieties of this species. 
Formation and locality.—Common near the top of the Trenton limestone at Minneapolis, Minnesota; 
and Decorah, Iowa. Also in the ‘‘ Lower Blue beds” of the Trenton at Janesville and Beloit, Wisconsin. 
“Tn certain beds of the Chazy limestone there are multitudes of a small Orthis which have, as nearly as I 
can judge, precisely the form and dimensions of 0. perveta, but, in consequence of their being imbedded 
in a rather compact subcrystalline rock, I have not been able to procure any specimens with the surface 
well preserved.” (Billings, Can. Nat. and Geol., vol. iv, p. 434.) These localities are two miles north of 
Montreal, Canada, and two or three miles west of Chazy, New York. This same form is believed to be 
the one identified by Mr. Walcott as O. perveta, which also occurs abundantly in the upper beds of the 
Pogonip group in Nevada. (Mono. U.S. Geol. Survey, vol. viii, p, 72.) 
Collectors.—O, L. Herrick, E. O. Ulrich and the writers. 
Mus. Reg. Nos. 651 (type specimen lost), 753, 5072, 5094, 7978-7982. 
Variety PERVETA Conrad. 
PLATE XXXIII, FIGS. 40—42. 
1843. Orthis perveta CONRAD. Proceedings of the Academy of Natural Sciences of Philadelphia, 
vol. i, p. 333. 
1847. Orthis perveta HALL. Paleontology of New York, vol. i, p. 120, pl. xxxu, fig. 5 
1859. Orthis perveta BILLINGS. Canadian Naturalist and Geologist, vol. iv, p. 434, fig. 10. 
1862. Orthis perveta HALL. Geology of Wisconsin, vol. i, p. 42, fig. 7. 
1863, Orthis perveta BILLINGS. Geology of Canada, p. 130, fig. 57. 
1880. Orthis media N. H. WiINcHELL. Bighth Annual Report of the Geological and Natural 
History Survey of Minnesota, p. 64. 
1880. Orthis kassube N. H. WINCHELL, Ibidem, p. 65. 
21884. Orthis perveta WaLcorr. Monograph of the U.S. Geological Survey, vol. viii, p. 72, pl. Xt, 
fig. 3. 
1892. Orthis (Dalmanella) perveta HALL. Paleontology of New York, vol. viii, pt. i, pl. vC, fig. 12. 
Original description: “Transversely oval, wider than the length of the hinge- 
line; valves slightly ventricose, subequal, with numerous prominent radiating stric, 
bifurcated on the umbo; larger valve ventricose in the middle, with a slight central 
depression; sides somewhat depressed; the opposite valve flattened towards the base 
and depressed to correspond with the elevation of the other valve, forming a sinuous 
margin when viewed in profile; base truncated; superior lateral margin obliquely 
truncated, rounded inferiorly. Length, one-third of an inch; breadth, nearly half 
an inch, 
“Locality: Mineral Point, Wisconsin, (Trenton limestone).” 
This variety can readily be distinguished by its coarser stria. The fold and 
sinus are also usually more pronounced than in 0. subequata, and occasionally are 
developed equally as strong as in var. gibbosa, when it is impossible to separate it 
from the latter. In Minnesota these shells are much distorted by pressure, a peculi- 
arity at once striking. Well preserved specimens, however, show close relationship 
