GENERA OF THE SUBORDER ORTHOIDEA 



135 



idomcllas the brachiophores become much curved and 

 extend ventrally rather than antero-ventrally as in the 

 more normal species. This increase in depth to form 

 globular shells among the late Paleozoic brachiopods 

 is also repeated in Entcletes and Meckella. 



Evidences of old-age features are to be seen in the 

 overgrowth of the myophore and the great develop- 

 ment of adventitious substance, especially about the 

 brachiophores, in the umbonal cavities, and in thick 

 deposits under the muscle fields in both valves. 



The origin of Rhipidomella is to be looked for in 

 Dalmanella of the Ordovician. In typical Dalmanelln 

 the diductors are long and slightly expanded in front, 

 and the adjustor scars outside of them are well devel- 

 oped and prominent. The adductor impressions are 

 central but not completely enclosed. In such a pattern 

 are the fundamentals of the Rhipidomella ventral mus- 

 culature. The transition is suggested by D. tersn, 

 which is structurally intermediate between Dalmanella 

 and Rhipidomella. Rhipidomella must have evolved 

 from Dalmanella in the late Ordovician, as there are 

 a number of early Silurian shells that already have the 

 typical characters of Rhipidomella. 



Subgenus PERDITOCARDINIA Schuchert 

 and Cooper 1931 



(Lat. perdere, to lose; car do, hinge) 



PI. 19, figs. 12, 14-17,20-22 



Schuchert and Cooper, Amer. Jour. Sci. (5), vol. 22, 1931, 

 p. 246. 



Genoholotype. — Orihis dubius Hall 1858, Trans. 

 Albany Inst., vol. 4, p. 12. 



Discussion. — In the St. Louis and Spergen lime- 

 stones of Indiana, Illinois, Missouri, and Kentucky, 

 there is a peculiar shell which is a distinct divergence 

 from the main line of Rhipidomella, enough so to 

 merit a subgeneric rank. This shell, Orthis duhia, 

 differs from Rhipidomella in not having interareas or 

 palintrope. The hinge-line has been narrowed to dis- 

 appearance, with consequent obsolescence of the inter- 

 areas. As a result, the shell is rostrate and suggests 

 Terebratulina in external appearance. So far has the 

 rostration gone that the ventral beak may be curved 

 over the dorsal, enclosing it and obscuring it from 

 view. With this rostration have also gone internal 

 changes. The teeth resemble those of Neothyris, being 

 attached to the floor and inner shell margin. In the 

 dorsal valve the cardinal process, even in young forms, 

 has become anchylosed with the brachiophores and 

 their supporting tissue. 



The rostration with resultant loss of interareas is not 

 a gerontic characteristic, but is well defined in the earli- 

 est stages, showing that the evolution is well established. 

 The one species thus far assigned to this subgenus is 

 a composite, and it will probably prove desirable to 

 separate it into several specific groups. 



Genus PLATYORTHIS Schuchert and Cooper 

 1931 



(Gr. platus, flat) 



PI. 19, figs. 18, 19,23,24,27-29,31 



Schuchert and Cooper, Amer. Jour. Sci. (5), vol. 22, 1931, 

 p. 246. 



Genoholotype. — Orthis planoconvexa Hall 1859, 

 Pal. N. Y., vol. 3, p. 168, pi. 12, figs. 1-6. 



Description. Exterior. — Subcircular to elliptical, 

 hinge-line narrow; lateral profile plano-convex, lateral 

 commissure straight; anterior commissure rcctimar- 

 ginate; no fold or sulcus; ventral umbo swollen, 

 interarea greatly reduced, nearly orthocline; beak 

 strongly incurved, usually closely appressed to the mar- 

 gin of the dorsal valve; delthyrium open; dorsal 

 interarea reduced nearly to disappearance ; ornamen- 

 tation multicostellate ; test fibrous, punctate. 



Ventral interior. — Delthyrial cavity deep; teeth 

 strong, elongate, standing as ridges on the dental 

 plates; crural fossettes deep; dental plates low and 

 stout, widely divergent, produced forward as sharp 

 ridges along the lateral margins of the diductor scars; 

 muscle field large, flabellate, occupying more than 

 half of the area of the interior; diductor scars elon- 

 gate, semiflabellate, centrally divided by a low ridge; 

 adductor scars elongate, oval, borne on a slight expan- 

 sion of the median ridge. 



Dorsal interior. — Cardinalia strong; brachiophores 

 produced into a point at the distal extremity, brachio- 

 phore supports as in Rhipidomella, divergent, vertical, 

 continued around the lateral margins of the muscle 

 area as a thick ridge; cardinal process ponderous, 

 ventro-centrally cleft, posteriorly bilobed but when 

 viewed from the ventral surface appearing to be tri- 

 lobed, its shaft produced for a short distance forward 

 as a median ridge, which does not extend to the an- 

 terior margin of the muscle area; muscle field sub- 

 quadrate, anterior adductor scars the larger. 



Geologic range. — Silurian to Middle Devonian. 



American Species 



Dalmanella lucia Chrke 1908 

 D. planoconvexa {Hi]\) 18 59 



European Species 



Orthis circularis Sowerby 1 842 

 O. opercularis Verneuil 1845 

 Dalmanella cimex Kozlowski 1929 



Distinguishing characters. — The chief distin- 

 guishing characters of Platyorthis are the plano-convex 

 profile, dalmanelloid exterior combined with ventral 

 muscle-scars and cardinalia like those of Rhipidomella, 

 and a huge cardinal process. The brevity of the hinge- 

 line, the lateral flaring of the dental plates, and the 



