418 THE PALEONTOLOGY OF MINNESOTA. 
[Orthis tricenaria. 
appearance in this genus, and especially in the typical species of the genus, is inter- 
esting, but it cannot be embraced in the diagnostic characters, since its presence 
appears to b6 largely of a specific value, and the degree of its development dependent 
upon the stage of growth, [For further remarks on this plate see DinoHhis.] 
"The muscular scar of the pedicle valve of 0. calligramma is a subelliptical area 
scarcely longer than the cardinal face, faintly impressed, and its components rarely 
distinguishable. 
"The structure of the shell is comparatively fibrous and impunctate. Specimens 
of Orihis calligramma, var. davidsoni, from Gotland, show openings of oblique tubules 
on the external surface, always situated upon the keels of the costae. These are 
sparse and irregularly scattered, but of similar nature to those seen in the impunc- 
tate species 0. subquadrata 'and the punctate species 0. suhcequata, 0. michelini. 0. 
resupinata and their allies." (Hall, op. cit.) 
Orthis tricenaria Conrad. 
PLATE XXXII. PIGS. 18-23. 
1843. Orthis tricenaria Conrad. Proceedings of the Academy of Natural Sciences of Philadelphia, 
vol. i, p 333. 
1843. Orthis disparilis Conrad. Ibidem, p. 333. 
1844. Orthis testudinariaf Owen. Geological Explorations in Iowa, Wisconsin and Illinois, pi. xv, 
flg. 11. 
1847. Orthis tricenaria Hall. Paleontology of New York, yol. i, p. 121. pi. xxxii, flg. 8. 
1847. Orthis disparilis Hall. Ibidem, p. 119, pi. xxxir, flg. 4. 
1859. Orthis disparilis Billings. Canadian Naturalist and Geologist, vol. iv, p. 440, flg. 20. 
1859. Orthis tricenaria Salter. Canadian Organic Kemains, dec. i, p. 39, pi. ix, flgs. 1-4. 
1862. Orthis tricenaria Hall. Geology of Wisconsin, vol. i. p. 42, flgs. 8-11. 
1862. Orthis disjmrilis Hall. Ibidem, p. 435. 
1863. Orthis disparilis Bix,-LiSQS. Geology of Canada, p. 130, flg 60. 
• 1863. Orth's tricenaria Billings. Ibidem, p. 167, flg. 151. 
1875. Orthis plicatella? White (not Hall). Report of the U. S. Geographical .^^urvey west of the 
100th Meridian, p. 72, pi. iv, flg. 10. 
1883. Orthis tricenaria Hall. Second Annual Report, N. Y. State Geologist, pi. xxxv, flgs. 1-5. 
1884. Orthis tricenaria Walcott. Monograph of the U. S. Geol. Surv., vol. viii, p. 74, pi. xi, flg. 4. 
1892. Orthis tricenaria Hall. Palajontology of New York. vol. viii, pt. i, pp. 191, 193, 221, 228, pi. v, 
flgs. 9-12. 
1892. Orthis disparilis Hall. Ibidem, pp. 191, 221, 228. 
Original description: "Semioval, with];about thirty prominent, very regular, 
rounded ribs; larger [ventrall valve ventricose; summit elevated; the dorsal margins 
subrectilinear, very oblique; lesser [dorsal] valve flat or slightly concave in the 
middle; cardinal area very wide; apex of the larger valve profoundly elevated above 
that of the opposite valve. Length, three-fourths of an inch." 
Shell semicircular in outline; plano-convex; anterior margin somewhat deflected 
ventrally. Hinge-line equal to the greatest width of the shell, rarely shorter. 
Cardinal area well developed on each valve, widest in the ventral valve, striated 
longitudinally and transversely, and divided by a very narrow delthyrium, which is 
