54 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



plates on the floor of the valve to form a pedicle spoon, 

 or whether the floor of the delthyrial cavity is covered 

 by a thick deposit of adventitious material between the 

 plates so that the ensemble resembles a pseudospondyl- 

 ium can not be proved definitely in the mature speci- 

 mens at hand. At any rate, the dental plates are 

 convergent and in this respect are very different from 

 those of Billingsella, which are very widely divergent. 

 The difficulty in defining precisely the structure of the 

 dental plates in Oligoniys is due to the fact that the 

 umbonal cavities are filled with adventitious material 

 at maturity. The ventral pallial trunks are almost 

 exactly like those of Billingsella in plan. However, the 

 antero-lateral and antero-median divergence of the 

 main trunk lines is rarely clearly visible. 



In the dorsal valve the structure of the cardinalia 

 is essentially the same as that of Billingsella, but there 

 are differences in detail. In the first place the dorsal 

 interarea is not so strongly apsacline as in the true 

 Billingsellas, with the result that the brachiophores are 

 more inclined. The cardinal process is usually thicker 

 in front and the notothyrial platform is likewise thicker 

 and the axial elevation more prominent. The dorsal 

 musculature is visible clearly in Oligomys exforrecta. 

 The anterior adductor scars present an elongate track, 

 but the posterior pair is a small impression outside the 

 plane of each of the anterior adductors. The scars 

 are separated by low oblique ridges in old shells. 



Family FINKELNBURGIID^, nov. 



(Finkelnburgiinje and Orusiinas Schuchert and 

 Cooper 1931) 



Primitive, costellate, biconvex Orthacea, having a 

 pseudospondylium and plectorthoid cardinalia. 



Geologic range. — Upper Cambrian to Canadian 

 of the United States, Canada, and western Europe. 



Includes but the two genera, Finkelnburgia and 

 Orusia, both of Walcott. 



Discussion. — Walcott says® that Finkelnburgia is 

 derived from Eoorthis. The present authors, on the 

 contrary, are inclined to the view that it had its origin 

 in Orusia and that it gave rise to Plectorthis and the 

 Plectorthidas. The origin of Orusia is unknown. 



Genus ORUSIA Walcott 1905 



PI. 1, figs. 2, 7, 9 



Walcott, Proc. U. S. Nat. Mus., vol. 28, 1905, p. 273; 

 Camb. Brach., 1912, p. 765, pi. 98, figs. 1-lp, 2-2k, 

 3-3b, 6-6c. 



Genoholotype. — Anomites lenticularis Wahlen- 

 berg 1821, Nova Acta Reg. Soc. Sci. Upsala, vol. 8, 

 pp. 66-67 (transl. in Matthew, Trans. Roy. Soc. 

 Canada for 1891, 1892, vol. 9, sec. 4, no. 5, p. 46). 



"Camb. Brach., p. 320 and table, p. 317. 



Description. Exterior. — Shell small, subelliptical 

 in outline; hinge-line narrower than the greatest 

 width of the shell; cardinal extremities usually 

 rounded. Lateral profile subequally biconvex; anterior 

 commissure sulcate or rectimarginate, sulcus most 

 strongly marked in the middle and dorsal regions of 

 the shell. Ventral interarea curved, moderately apsa- 

 cline, delthyrium open. Dorsal interarea shorter than 

 the ventral, anacline. Shell structure unknown. 



Ventral interior. — Delthyrial cavity shallow; teeth 

 small, dental plates divergent, sharply defined ; muscle 

 area confined; adductor track triangular; diductor 

 tracks narrow, continuous with the pallial marks; 

 pallial trunks widely divergent, as in Billingsella. 



Dorsal interior. — Notothyrial cavity shallow ; brach- 

 iophores long and sharp, continuous with sharply 

 defined subparallel plates which extend nearly vertically 

 to the roof of the valve; cardinal process absent. 



Geologic range. — Upper Cambrian to Canadian 

 (according to Bassler 1915). 



American Species 



Ort/tis lenticularis (Wahlenberg) 1876 

 O. lenticularis atryfoides Matthew 1 892 

 O. lenticularis lyncioides Matthew 1892 

 Rafinesquina (?) atava (Matthew) 1893 

 Eoorthis johannensis (Matthew) 1892 



European Species 

 Orthis lenticularis (Wahlenberg) 



Discussion. — The genus Orusia is made up of 

 small, thin-shelled species characterized by a hinge that 

 is narrower than the width of the valves; sharp, diver- 

 gent, thin dental plates; and short, discrete, subparallel 

 brachiophore plates that are thin and delicate. 



The essential structure of the ventral valve is like 

 that of Billingsella, having similar pallial markings and 

 musculature, but is close to Finkelnburgia in its dental 

 lamellae. In the dorsal valve, however, the delicacy 

 of the brachiophore plates, together with the fact that 

 the trace of these plates on the internal molds is sub- 

 parallel, makes them unique among Cambrian brachio- 

 pods and relates them to Finkelnburgia. It has not 

 been possible to determine their exact nature but they 

 extend with a slight curvature to the floor of the valve. 

 The brachiophores, so far as can be observed, are rather 

 long and sharp. The structure of the cardinalia is 

 very close to that of Finkelnburgia. The musculature 

 of the dorsal valve is not known. 



The type of cardinalia in Orusia is unique among the 

 known Cambrian orthid brachiopods. In the Billings- 

 ellidas and Nisusiidas, support of the brachiophores is 

 accomplished by the swelling of the notothyrial plat- 

 form laterally and ventrally, so that the plates are 

 buttressed by a foundation of callus. In Orusia the 

 advent of discrete plates for the support of the brachio- 



