86 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



Genoholotype. — Orthls tricenaria Conrad 1843, 

 Proc. Acad. Nat. Sci. Philadelphia, vol. 1, p. 333, as 

 described by Hall and Clarke (1892). 



Description. Exterior. — Semicircular to semi- 

 elliptical; hinge-line straight; cardinal extremities 

 acutely angular or a right angle. Lateral profile plano- 

 or slightly concavo-convex ; anterior commissure recti- 

 marginate or faintly and broadly sulcate. Ventral in- 

 terarea long, plane or gently curved, apsacline, umbo 

 convex, beak gently curved, delthyrium long and nar- 

 row, subtending an angle of 30° or less, partially or 

 completely covered by a deltidium. Dorsal interarea 

 shorter than the ventral, plane, strongly anacline, noto- 

 thyrium open or partially closed by a complete chil- 

 idium. Ornamentation paucicostate, interspaces with 

 parvicostellae and concentric elevated growth-lines. 

 Test fibrous, impunctate, external surface in some 

 species with distantly spaced, coarse pits. 



Ventral interior. — Delthyrial cavity deep; teeth 

 small, crural fossettes shallow, oblique; dental plates 

 receding; margin of the delthyrium formed by the 

 palintrope or median extensions thereof; muscle field 

 subcircular or subcordate ; diductor scars subcrescentic 

 or subtriangular; adductor marks central, lanceolate, 

 not extending anterior to the ends of the diductor im- 

 pressions; adjustor scars not observed. Two sub- 

 parallel ridges extend from the ends of the diductor 

 scars nearly to the front of the shell. Between these 

 is a faint ridge extending from the anterior margin 

 of the adductors. In some specimens this is forked 

 at its distal extremity. Ovarian impressions occupy the 

 internal lateral spaces and umbonal cavities, being also 

 visible on the lower (anterior) surface of the palin- 

 trope. Ovarian markings separated by subparallel pal- 

 lial sinuses as in Orthis s. s. Anterior margin crenu- 

 lated, each internal rib being cleft medianly. 



Dorsal interior. — Notothyrial cavity shallow; car- 

 dinalia confined; brachiophores simple, ventrally cari- 

 nate, forming the margins of the notothyrial cavity, 

 bearing blunt points from the dorso-distal extremity. 

 Brachiophores unsupported except by the shell sub- 

 stance of the notothyrial platform, which is a promi- 

 nent posterior thickening of the valve. Cardinal 

 process a simple ridge expanded slightly and not 

 uncommonly cleft anteriorly; a broad median ridge 

 extends from the notothyrial platform to the anterior 

 margin of the muscle area. Muscle area large, an- 

 terior adductors triangular, larger than the posterior 

 pair. Ovarian markings occupying the internal lateral 

 spaces. Anterior margin as in the ventral valve. 



Geologic range. — Middle Ordovician (Chazy) 

 to at least the middle of the Upper Silurian. 



American Species 



Orthis apicaUs Billings 1865 

 O.costalis Hall 1847 

 O. JavidsoniVerneuW 1848 

 O. disfarilis Conrad 1843 



O. fyramidalis Twenhofel 1928 



O. tricenaria Conrnd. 1843 



Billingsella (?) laurentina (Billings) 1857 



European Species 



Orthis inostrantzefi Wysogorsky 1900 

 O. inostrantzefi ubjaensis Opik 1930 

 O. inostrantzefi viruana Opik 1930 

 O.flicata Sowerby 1839 



Discussion. — Hall and Clarke in their revision of 

 the American brachiopod genera designated a "Group 

 of Orthis callactis" as the restricted Orthis. Before 

 them, however, Davidson had selected the European 

 O. callactis as the type of the genus. It is clear that the 

 American authors in restricting the current conception 

 of Orthis had in mind O. tricenaria Conrad of the 

 Black River and Trenton of the eastern and mid- 

 western United States as the genotype. Since there 

 are very important differences between their "Euro- 

 pean" and "American" groups of "Orthis," it is now 

 necessary to designate the latter by a new name, 

 Hesferorthis, or the Orthis of the West. 



Important differences between these two groups are 

 to be found both inside and out. Externally the Euro- 

 pean Orthis s. s. is characterized by a short, strongly 

 curved ventral interarea, incurved beak, and swollen 

 umbo, features seen in the North American forms in 

 only two species, O. fanderiana and O. buttsi, n. name. 

 The American or "western" Orthis, on the other 

 hand, is distinct by virtue of its elongate interarea, 

 slightly incurved beak, and gently convex umbo. 



The markedly different outside expressions of the 

 two groups have made for certain internal differences. 

 It has been shown already that the dental plates of 

 Orthis s. s. are extended ventrally directly to the floor 

 of the valve, bounding rather shallow umbonal cavi- 

 ties, which, in mature and gerontic individuals, may 

 become obsolete by deposition in them of adventitious 

 shell. In Hesperorthis, on the other hand, the dental 

 plates, as is usual in shells with long and broad palin- 

 tropes, are receding, that is, extend as ridges postero- 

 ventrally for some distance before uniting with the 

 floor of the valve. The muscle patterns and pallial 

 impressions in the ventral valves of the two groups are 

 essentially alike, thus indicating their close family con- 

 nections, and their probable origin in some eoorthoid 

 ancestor. 



In the dorsal interiors of the two genera are to be 

 found other significant variations. The brachiophores 

 in Hesferorthis are usually more elongate and are 

 drawn out into sharp points distally. Further, the 

 muscle field is larger and the anterior adductor scars 

 are subtriangular. 



There is still another, perhaps more striking, diflFer- 

 ence between these two groups, which is the presence 

 of a partial or a fully developed deltidium and chil- 

 idium in the American forms and the total absence of 

 these features in the European shells. The only group 

 of Russian orthids showing any modification of the 



