206 
PALAEONTOLOGY OF NEW YORK. 
1875. Orthis, White. Geogr. and Geol. Expl. west 100th Merid., pp. 55, 72. 
1878. Orthis, Calvin. Bull. U. S. Geol. and Geogr. Surv Terr., vol. iv., p. 728. 
1879. Orthis, Hall. Twenty-eighth Rept. N. Y. State Mus. Nat. Hist., p. 150, pi. xxi, figs. 11-17. 
1880. Orthis, N. H. Winchell. Eighth Ann. Rept. Geol. Nat. Hist. Surv. Minnesota, pp. 63, 64. 
1882. Orthis, Whitfield. Geology of Wisconsin, vol. iv, p. 258, pi. xii, figs. 5-7 and p. 320, pi. xxv, 
figs. 3, 4. 
1882. Orthis, Hall. Eleventh Rept. State Geologist of Indiana, p. 285, pi. xxi, figs. 11-17. 
1883. Orthis, Hall. Report N. Y. State Geol. for 18S2, pi. xxxiv, figs. 1-24 and pi. xxxv, figs. 23-27. 
1884. Orthis, Walcott. Palaeontology Eureka Dist., p. 72, pi. xi, figs. 3, 10. 
1885. Oi'this, Foerste. Bull. Denison University, vol. i, pin S4, 85, pi. xii, fig. 11; pi. xvii, fig. 13. 
1889. Orthis, Nettleroth. 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. Orthis, Beecher and 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 the 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 lamellae which are produced forward circumscribing 
a rather short suboval or subquadrate muscular area, which is more or less dis¬ 
tinctly defined in different 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 
