. Brachiopod genera of the suborders Orthoidea and Pentameroidea. Orthoidea; Pentameroidea; Paleontology -- Paleozoic. GENERA OF THE SUBORDER ORTHOIDEA 55 phores is a great advance over that of other Cambrian forms. Cumings'" has suggested the origin of Platystrofhia from Orthis leniicularis {=Orusm). There appear to be no anatomical difficulties in the way of this evolu- tion, yet it is impossible to reconcile with this idea the geological position of these genera in time. Orusui is an Upper Cambrian and possibly a Canadian genus; PLitystroph'w, on the other hand, is known first from the
. Brachiopod genera of the suborders Orthoidea and Pentameroidea. Orthoidea; Pentameroidea; Paleontology -- Paleozoic. GENERA OF THE SUBORDER ORTHOIDEA 55 phores is a great advance over that of other Cambrian forms. Cumings'" has suggested the origin of Platystrofhia from Orthis leniicularis {=Orusm). There appear to be no anatomical difficulties in the way of this evolu- tion, yet it is impossible to reconcile with this idea the geological position of these genera in time. Orusui is an Upper Cambrian and possibly a Canadian genus; PLitystroph'w, on the other hand, is known first from the Trenton in this country. There is thus a vast time gap between the disappearance of Orusut and the ap- pearance of Platystroph'ui, a time involving at least all of the Chazyan. The ancestor of Platystrofhia should be looked for in the Chazyan rather than the Cambrian or Canadian. It appears to us, however, that Orusia is ancestral to Fittkelnhurgia and perhaps thus directly in the plectorthid line which, as we interpret the facts, gave rise to Platystrofhia. Genus FINKELNBURGIA Walcott 1905, emended PI. 13, figs. 6, 7, 9-19; t. fig. 16 Walcott, Proc. U. S. Nat. Mus., vol. 28, 1905, p. 277; Camb. Brach., 1912, p. 793, pi. 93 (we would select figs. 2 and 2e as representing the genotype). Genoholotype.—Finkelnburgia finkeltiburgiWal- cott 1905. Description. Exterior.—Subsemicircular to sub- elliptical; hinge-line straight; cardinal extremities acute or obtuse; lateral profile subequally to unequally biconvex. Anterior commissure rectimarginate. Ven- tral palintrope longer than the dorsal one, apsacline; delthyrium and notothyrium open, so far as revealed by the specimens studied, but Walcott erroneously states that the delthyrium is partially covered by a del- tidium. Surface finely multicostellate. Test fibrous, impunctate. Ventral interior.—Delthyrial cavity deep; teeth strong; dental plates receding; a thickening under the muscles forms a pseudospondylium which is pro- d
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