418 THE PALEONTOLOGY OF MINNESOTA. 



(prtliis trlcenarm. 



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 be largely of a specific value, and the degree of its development dependent 

 upon the stage of growth. [For farther remarks on this plate see Dinorthis.] 



"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 Orthis calligramma, var. davidsoni, from Gotland, show openings of oblique tubules 

 on the external surface, always situated upon the keels of the costse. 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. subcequata, 0. michelini, 0. 

 resupinata and their allies.'* (Hall, op. cit.) 



ORTHIS TEICENARIA Conrad. 



PLATE XXXII, FIGS. 18-23. 



1843. Orthi* Mcenaria 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, 



fig. 11. 



1847. Orthis tricenaria HALL. Palasontology of New York, vol. i, p. 121, pi. xxxu, fig. 8. 



1847. Orthia disparilis HALL. Ibidem, p. 119, pi. xxxn, fig. 4. 



1859. Orthis disparilis BILLINGS. Canadian Naturalist and Geologist, vol. iv, p. 440, fig. 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 disparilis HALL. Ibidem, p. 435. 



1863. Orthis disparilis BILLINGS. Geology of Canada, p. 130, flg 60. 

 1863. Orth's tricenaria BILLINGS. Ibidem, p. 167, flg. 151. 



1875. Orthis plicatellat WHITE (not HALL). Keport of the U. S. Geographical Survey 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. Suiv., vol. viii, p. 74, pi. xi, flg. 4. 

 1892. Orthis tricenaria HALL. Paleontology of New York, vol. viii, pt. i, pp. 191, 193, 221, 228,'pl. 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 [ventral] 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 



