450 THE PALEONTOLOGY OF MINNESOTA. 



[Variety pcrvcln . 



divaricator process was seen in any silicified specimens observed. In 0. subcequata, 

 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 0. 

 subcequata, 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. 

 " In 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, fn 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. C. L. Herrick, E. O. Ulrich and the writers. 



Mm. Reg. Nos. 651 (type specimen lost), 753, 5072, 5094, 7978-7982. 



Variety PEEVETA Conrad. 



PLATE XXXIII, FIGS. 4042. 



1843. Orthis perveta CONRAD. Proceedings of the Academy of Natural Sciences of Philadelphia, 



vol. i, p. 333. 



1847. Orthis perveta HALL. Palaeontology of New York, vol. i, p. 120, pi. xxxn, 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. WINCHELL. Eighth Annual Report of the Geological and Natural 



History Survey of Minnesota, p. 64. 

 1880. Orthis kassubce N. H. WINCHELL, Ibidem, p. 65. 

 91884. Orthis perveta WALCOTT. Monograph of the U. S. Geological Survey, vol. viii, p. 72, pi. xi, 



flg. 3. 

 1892. Orthis (Dalmanella) perveta HALL. Paleontology of New York, vol. viii, pt. i, pi. vC, flg. 12. 



Original description: "Transversely oval, wider than the length of the hinge- 

 line; valves slightly ventricose, subequal, with numerous prominent radiating striae, 

 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 striae. The fold and 

 sinus are also usually more pronounced than in 0. subcequata, 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 



