BRACHIOPODA. 431 



On], is (Dlnorthlg) proavita .1 



beyond the early nealogic stage. Here, then, is a reversion to ancestral characters, 

 which is carried still further in 0. flabellulum Hall* (non Sowerby)=0. 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, Iowa, and at Iron Ridge, Wisconsin. 



Collectors. E. O. Ulrich, John Kleckler and the writers. Also in the collection of Dr. C. H. Robbins, 

 of Wykoff, Minnesota. 



Mus. Reg. Nos. 274, 396, 4076, 4094, 7786-7788. 



ORTHIS (DINORTHIS) PROA VITA W. and S. 



PLATE XXXII, FIGS. 51-57. 



1892, April 1. Orthis proavita W. and S. American Geologist, vol. ix, p. 290. 

 1892, April 9. Orthis petrce SARDESON. Bulletin of the Minnesota Academy of Natural Sciences, 



vol. iii, p. 332, pi. v, figs. 18-21. 



Shell of medium size; subqaudrate; 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 striae 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 striae 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 0. 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. 



0. (D.) proavita differs from 0. (D.) iphigenia Billings,! in having the fold and 

 sinus reversed; the latter also has a greater number of striae and is restricted to the 



*Geol. N. Y.; Eep. Fourth DIst.. p. 105, flg. 5, 1843: Pal. N. Y., vol. il, pp. 254, 255, pi. XMI, fl s s. 6, 7, 1W>2. 

 ** Proc. Boston Soc. Nat. Hist., vol. xxl v, p 308, pi. vi. flgs. 4, 5, 1890. 

 (Pal. Foss.. vol. i, p. 1*3. tig. 110. 1862' 



