BRACHIOPODA. 261 



Genus DERBY A, Waagen. 1884. 



I'LATE X, FIGS. 10-17; PLATE XI, FIGS. 1-5, lS-22; PLATE XI,V, FIGS. •i'!-:!i); PLATE XIli, FIGS. 1-8, 10, 17, 2:!, '.M; 



AND PLATE XX. 



1836. Spirife): Phillips. IlUistr. Geolog-y of Yorkshire, vol. ii, jil. ix. tig-. .'>. 



1844. Orthis, McCoy. Synop-sis Carbonif. Fossils of Shetland, p. 123, j)l. xxii, fig-. 1. 



1852. Orthi.i, Owen. Geological Rept. of Wisconsin, Iowa and Minnesota, pi. v, fig. 11. 



1853. Orthisina (partim), Davidson. Introd. British Fossil Brachiopoda, p. 104. 

 1855. LeptcBna, McCoy. British Palseozoic Fossils, p. 4.02. 



1858. Orthis, Hall. Geology of Iowa, pt. ii, p. 640, pi. xix, iig. 5 ; p. 173, pi. xxviii, tig. 5. 



1858. O/'iMwia, Meek and Haydbn. Proc. Aoad. Nat. Sciences Phila.. vol. x, p. llil. 



1860. Orthis, McChesney. New Palaeozoic Fossils, p. 32. 



1861. Streptorhynchus (partim), Davidson. British Carb. Brachiopoda, p. 124, pi. xxviii, figs. 2-7, 0. 

 1864. Hemipronites, Meek and Haydbn. Rept. Pal.-eont. Upper Missouri, p. 26, pi. i, fig. 7. 



1868. Hemipronites, McChesney. Trans. Chicago Academy Sciences, p. 28, pi. i, figs. 5, 6. 



1872. Hemipronites, Meek. Rept. PalaBOntology Eastern Nebraska, p. 174, pi. v, fig. 10 ; pi. vii, tig. 1. 



1873. Hemipronites, Meek and Wobtiien. Geol. Snrv. Illinois ; Pah^ont., vol. v, p. 570, pi. xxv, tig. 12. 



1874. Streptorhynchus, Derby. Carboniferous Brachiopoda of Itaitulia, p. 32, pis. vii, viii. 



1878. Streptorhynchus, Abich. Bergkalkfauna aus der Araxesenge bei Djoulfa in Armenien, p. 73. 



1883. Streptorhyncli us. Hall. Second Ann. Rept. N. Y. State Geologist, pi. xl, tigs. 10-17; pi. xli, 



figs. 1-3, 18-22. 



1884. Derhyia, Waagen. Palaeontologica Indica, Ser. xiii, vol. i, pt. iv, pp. 576, 591-607, pi. Ii, fig. 1 ; 



111. Hi, figs. 1-3 ; pi. liii, tigs. 1-5 ; pi. liv, figs. 1-4 ; pi, Iv, fig. 3. 

 1887. Derbya, ffiHLERT. Fischer's Manuel de Conchyliologie, p. 1285. 

 1887. Hemipronites, Herrick. Bull. Denison University, vol. ii, p. 50, pi. 2, fig. 19. 



Diagnosis. General character of the shell as in Orthothetes. The external 

 form is more variable, being usually plano-convex, rarely concavo-convex, the 

 pedicle-valve often more elevated and subpyramidal, becoming irregular in its 

 growth toward the apex. The interior of the pedicle-valve bears a high median 

 septum, extending longitudinally through the center of the muscular area, 

 which is from one-third to two-thirds the length of the valve ; near the apex 

 of the umbonal cavity it unites wdth the dental plates forming a small trian- 

 gular chamber beneath the deltidium. In old shells this cavity is often filled 

 by a testaceous secretion. The dental plates do not extend to the bottom of the 

 rostral cavity except near the apex. 



Brachial valve broad and regularly convex, or sometimes with an undefined 

 median depression, and showing a tendency to auriculation. Cardinal area 

 linear; the crural plates are abruptly elevated, and unite to form a strong 

 erect cardinal process, which is distinctly bilobed at the summit ; each lobe 

 being slightly notched at the summit and distinctly grooved along the posterior 

 face, the grooves uniting in a wider one at the posterior base of the process. 

 Midway in their length these crural plates are marked by a transverse ridge 



