BRACHIOPODA. 261 



Genus DERBY A, Waacen. 1884. 



I'LATK X, FIUS. 10-17; I'J-ATK XI, KIGS. 1-5, l*-2-2; PLATE XIa, KIGS. SKil); i-I.ATE Xlis, FIGS. 1-8, IG, 17, 'if, 24; 



AND PLATE XX. 



1836. Spirifer, Phillips. Illutitr. Geolofjy of Yorkshire, vol. ii, yil. ix, f\g. 5. 



1844. Oiihij), McCoy. Synopsis Carboiiif. Fo.'ssils of Shetland, p. 123, pi. xxii, fijf. 1. 



1852. Oi-thh; Owbn. Geological Rept. of Wisconsin, Iowa ami Minnesota, pi. v, fig. 11. 



ISSS. Orthhina ipartim), Davidson. Introd. British Fo.ssil Brachiopoda, p. 104. 



1855. Leptmna, McCoy. British Paleozoic Fo-ssils, p. 452. 



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



1858. Orthhina, Meek and Hatden. Proc. Acad. Nat. Scienc'es Phila.. vol. x, p. 161. 



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



1861. Streptorhynchu.1 {partim}, Davidson. British Carb. Brachiopoda, p. 124, pi. xxviii, tigs. 2-7, fl. 

 1864. Hemijjronites, Meek and Hayden. Rept. Palreont. Upper Mis.sonri, p. 26, pi. i. tig. 7. 



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



1872. HcmipronUeis, Meek. Rept. Palaeontology Eastern Nebraska, p. 174, pi. v, fig. 10 ; pi. vii, fig. 1. 



1873. Hciiiipronites, Meek and Worthen. Geol. Surv. Illinois ; Pahnont., vol. v, p. 570, pi. xxv, fig. 12. 



1874. Streptnr?/ynchns, Derby. Carlioniferous Brachiopoda of It.aitulia, p. 32, pis. vii, viii. 



1878. StreptorliyncliKS, Abich. Bergkalkfauna ans der Araxesenge bei Djoulfa in Annenien, p. 73. 



1883. jStreptorhynckiui, il.Ki.h. Second Ann. Rept. N. Y. State Geologist, ]>1. xl. tig.s. 10-17; pi. xli, 



figs. 1-3, 18-22. 



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



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

 1887. Derbya, CEulert. Fischer's Mannel rle Conchyliologie, p. 1285. 

 1887. Htmipronites, 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 with the dental plates forming a small trian- 

 gular chamber beneath the deltidiuni. 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 



