196 PALEONTOLOGY OF NEW YORK. 



is an erect apophysis, broadened and frequently bilobed on its summit and 

 posterior face. 



The shell-structure, like that of Orthis callacUs and 0. plicatella, is compactly 

 fibrous, and in all the species examined, impunctate. No evidence of tubulose 

 plications has been seen. 



In American faunas this is a small section limited to a few Silurian species, 

 such as 0. pedinella, Conrad, and 0. Sweeneyi, N. H. Winchell, from the Trenton 

 limestone, and it is closely allied in some general respects to the typical mem- 

 bers of Group V, but the minor differences will be found of permanent value. 



The other impunctate reversed shells may be separated into two groups, 

 first, — 



PL^SIOMYS 



(nom. propos). 



IV. Group of Orthis subquadrata. Hall. 



PLATE Va, figs. 14-21, 28-34. 



1843. Strophomena, Conrad. Proc. Acad. Nat. Sci., Phila., vol. i, p. 332. 



1847. Orthis, LepUBiia, Hall. Palaeontology of N. Y., vol. i, pp. 113.126, pi. xxxiia, fig-. 1 ; pi. xxxib, 



figs. 5, 6. 



1858. Orthis, Saltee. Mem. Geol. Surv. Great Britain, vol. ii, p. 373, pi. xxvii, figfs. 3, 4. 



1859. Orthis, Billings. Canadian Nat. and Geol., vol. iv, p. 438, fig. 15. 

 1859. Strophomena, Hall. Twelfth Kept. N. Y. State Cab. Nat. Hist., p. 70. 

 1861. Orthl% Hall. Thirteenth Rept. N. Y. State Cab. Nat. Hist., p. 120. 



1861. Orthis, McChe.snby. Descriptions New Palseozoic Fossils, p. 78. 



1862. Orthis, Hall. Geology of Wisconsin, p. 54, fig. 1, 2. 



1862. Orthis, Strophomtna, Billings. Palseozoic Fossils, vol. i, p. 130, fig. 108; p. 133, fig. 1 10 ; p. 135, 



fig. Ill, and p. 136, figs. 112, 113. 



1863. Orthis, Billings. Geology of Canada, p. 129, fig. 54 ; p. 165, fig. 146, and p. 312, fig. 319. 

 1873. Orthis, Meek. Palaeontology of Ohio, vol. i, pp. 92, 94, pi. ix, fig. 2, and pi. xi, fig. 7. 

 1875. Ortliis, Millkb. Cincinnati Quart. Jour. Sci., vol. ii, pp. 37, 38. 



1880. Ortliis, White. Second Ann. Rept. Indiana Bureau Statistics and Geol., p. 484, pi. i, figs. 3-5. 



1881. Orthis, White. Tenth Rept. Indiana State Geol., p. 116, pi. i, fig.s. 3-5. 

 1SS3. Orthis, Hall. Rept. N. Y. State Geol. for 1882, pi. xxxiv, figs. 28, 29. 

 1887. Orthis, Shaleb. Mem. Kentucky Geol. Surv., pp. 19, 22, pis. vi, vii. 



It may be difficult to point out external characters in which the shells of 

 this section differ from those of Group V. In Orthis subquadrata there is the 

 same general expression in contour and surface, but tlie reversal in the con- 

 vexity of the valves, and the retrorsion of the umbo of the pedicle-valve is 



