206 PALEONTOLOGY OF NEW YORK. 



1875. Ortlns, Whitk. Geogr. and Geol. Expl. west ICOth Meiid., pp. 55, 72. 



1878. OrtMs, Calvi.s. Bull. U. S. Geol. and Geoerr. Surv Ten-., vol. iv., p. 728. 



1879. Orthis, Hall. Twenty-eighth Kept. N. Y. State Mus. Nat. Hist., p. 150, pi. xxi, tij,^s 11-17. 



1880. OriJius, N. H. Winchbil. Eig-hth Ann. Rept. Geol. Nat. Hist. Sui-v. Minnesota, pp. G3, 64. 

 1882. Orthis, Whitfield. Geolog-y of Wisconsin, vol. iv, p. 258, pi. xii, tigs. 5-7 and p. 320, pi. xxv, 



fiys. 3, 4. 



1852. Orthis, H.tLL. Eleventh Rept. State Geologist of Indiana, p. 285, pi. xxi, figs. 11-17. 



1853. OrtJiis, Hall. Report N. Y. State Geol. for 1882, pi. xxxiv, tigs. 1-24 and pi. xxxv, figs. 23-27. 

 1SS4. OrtJiis, Walcott. Paleontology Eureka Dist., p. 72, pi. xi, figs. 3, 10. 



1885. Orthis, Foerstk. Bull. Deni.son University, vol. i, pp. 84, 85, pi. xii, fig. 11 : pi. xvii, fig. 13. 

 1889. Orthis, Nettlbroth. Kentucky Fossil Shells, p. 37, pi. xxxii, figs. 55-57. 

 1889. Orthis, Whitfield. Bull. American Mus. Nat. Hist., vol. ii. p. 43, pi. viii, fig. 1-4. 

 1889. Orthu, Bbecher ami Clarke. Mem. N. Y. State Mus. Nat. Hist., vol. i. No. 1, p. 14, pi. i, figs. 



3-11. 



Shells plano-convex or subequally biconvex. Pedicle-valve usually the 

 deeper, often gibbous, elevated at the umbo and arched over the cardinal area. 

 Hinge-line generally shorter than the greatest width of the shell. In many 

 of tlie species there is a more or less conspicuous, undefined median fold and 

 sinus on the pedicle and brachial valves respectively. Surface covered with 

 fine, rounded bifurcating striae. 



In the pedicle-valve the teeth are quite prominent, thickened at their ex- 

 tremities and supported by lamellas which are produced forward circumscribing 

 a rather short suboval or subquadrate muscular area, which is more or less dis- 

 tinctly defined in diiferent species and in different conditions of the shell. In 

 Orthis Meeki, Miller, a somewhat ponderous, biconvex, multistriate variation of 

 Orthis testudinaria, it is clearly resolvable into adjustor and diductor scars, the 

 latter bounding, but not altogether enclosing the impression of the adductors; 

 the pedicle-scar is also discernible. In the brachial valve the cardinal process 

 extends forward as a ridge to the bases of the crural plates, where it is broadened 

 and continued thence as a median ridge separating the muscular impressions. 

 The inner surface of this process is divided by a faint median furrow which 

 produces two lobes at the posterior extremity, and each of these lobes is again 

 divided, making the process quadrilobate. Sometimes the inner divisions of 

 the two main lobes have coalesced, producing a strong median lobe and thus 

 making the process appear trilobate. In some species at maturity, and in 

 others from abnormal growth this process becomes a broad plug, which fills 

 the entire delthyrial opening. The dental sockets are small, the crural plates 



