BRACHIOPODA. 431 
Orthis (Dinorthis) proavita .] 
beyond the early nealogic stage. Here, then, is a reversion to ancestral characters, 
which is carried still further in 0. flabellulum Hall* (non Sowerby)—0O. flabellites 
(Hall) Foerste,** a species from the Niagara formation much resembling 0. pectinella 
Emmons. 
Formation and locality.—This is a very characteristic and common species in America, everywhere 
marking the upper portion of the Hudson River group. In Minnesota it has been found abundantly at 
Spring Valley, and is known to be equally common at Wilmington, Illinois; Richmond, Weiseburg and 
Madison, Indiana; Oxford, Clarksville and Waynesville, Ohio; Maysville, Kentucky; and Anticosti. It 
also occurs rarely at Graf, [owa, and at lron Ridge, Wisconsin. f 
Collectors.—E. O. Ulrich, John Kleckler and the writers. Alsoin the collection of Dr. C. H. Robbins, 
of Wykoff, Minnesota. 
Mus. Reg. Nos. 274, 396, 4076, 4094, 7786-7788. 
Ortuis (Drnorrats) proavita W. and VS. 
PLATE XXXII, FIGS. 51—57. 
1892, Aprill. Orthis proavita W.and-S. American Geologist, vol. ix, p. 290. 
1892, April 9. Orthis petree SARDESON. Bulletin of the Minnesota Academy of Natural Sciences, 
vol. iii, p. 332, pl. v, figs. 18-21. 
Shell of medium size; subgaudrate; hinge-line equal to, or less than, the greatest 
width of the shell; cardinal anlges rounded or rectangular; sides gently convex and 
converging to more or less straghtened or slightly concave anterior margin. Surface 
marked by simple subangular strie having their origin at the apex of the valves or 
immediately below it, addition taking place by interpolation on the dorsal, and by 
bifucation on the ventral valve; one to three strize terminating on the cardinal mar- 
gin on each side of the umbo; thirty-seven to forty-two on mature examples, crossed 
by a variable number of imbricating growth lines near the anterior margin, In some 
specimens this margin is sharply reflexed, partly indicating old age. 
Ventral valve slightly elevated at the umbo, flattened or somewhat concave 
toward the lateral and anterior margins, with an insignificant mesial elevation. 
Area comparatively narrow, with a broad, triangular delthyrium, two-thirds of 
which is occupied by the cardinal process of the other valve; beak slightly incurved. 
Interior characters as in O, subquadrata, with the muscular scars remarkably well 
defined for a specimen of medium size. 
Dorsal valve more or less strongly convex, with the greatest elevation about 
mid-length. A shallow, or sometimes well pronounced, broad sinus is present, 
having its origin in the upper third of the valve. Area narrow, perpendicular or 
slightly inclined forward, with a broad delthyrium entirely occupied by the protrud- 
ing, striated cardinal process. 
O. (D.) proavita differs from O. (D.) iphigenia Billings,t in having the fold and 
sinus reversed; the latter also has a greater number of strize and is restricted to the 
*Geol. N. Y.; Rep. Fourth Dist.. p. 105, fig. 5, 1843: Pal. N. Y., vol. ii, pp. 254, 255, pl. x1, figs. 6, 7, 1852. 
** Proc. Boston Soc. Nat. Hist., vol. xxiv, p 308, pl. vr. figs. 4, 5, 1890. 
+ Pal. Foss., vol. i, p. 133, fig. 110, 1862 
