96 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



From O. ■pectinella we pass to the variety sweeneyi, 

 which is a local variation of it. Associated with the latter 

 are specimens in which the strong plications begin to divide 

 near the anterior margin. At the base of the Galena shales 

 the strong simple, plicated forms become rarer, while those 

 with more numerous striae prevail. Upon reaching the 

 strata containing Clitambonites diversa Shaler, provision- 

 ally known to the survey as Galena shales, the numerously 

 striated form, here described as O. meedsi, is the only one 

 found. Ascending into the Galena formation for thirty or 

 forty feet more we find O. meedsi still exhibiting a tend- 

 ency to increase the number of its striae [costella:] , and 

 finally assuming characters (variety germana) which attain 

 their greatest development in O. subquadrata of the Hudson 

 River [Richmond] formation. The change from O. fec- 

 tifiella to O. subquadrata is thus completed. 



Consistent with this argument they placed Plcesiomys 

 in the synonymy of Dinorthis and the same relationship 

 was continued by Schuchert in 1897 in the Synopsis 

 of American Fossil Brachiopoda.®^ It is, however, 

 doubtful that such a gradation between D. fectinella 

 and Plcesiomys subquadrata actually exists because in 

 the Chazy (possibly Black River) of Tennessee the 

 Dinorthis s. s. of that region are already showing a 

 marked divergence toward Pla-siomys. The D. trans- 

 versa, D. quadriflicata, and D. interstriata all have 

 interstitial or dichotomous ribs. Furthermore, the P. 

 ifhigenia of the Black River and P. ulrichi of the 

 Trenton are well established species agreeing in form 

 and ornamentation with O. subquadrata. 



There is still another angle at which this problem 

 of the distinction between Dinorthis and Plcpsiomys 

 may be viewed, and that is, that D. fectinella and 

 related forms represent the culmination of a trend 

 which went in the direction of simplification of ribs. 

 If this be true, some early Pleeslomys would be the 

 ancestor of Dinorthis. 



The value of the name Pleeslomys has been variously 

 interpreted. In 1911 Raymond^^ found it "best to 

 retain the name Pleeslomys for the present, and apply 

 it to such forms as O. subquadrata and O. flatys [our 

 Multlcostella, described farther on], in which the sur- 

 face is not coarsely plicated, but is covered with stria- 

 tions [costellas] which increase by bifurcation and im- 

 plantation." In 1913 Schuchert^^ regarded Dinorthis 

 as a subgenus of Pleeslomys, but later reversed the 

 relationship. 



It has been shown above that internally Dinorthis 

 and Pleeslomys are inseparable and should be regarded 

 as forming a single genus. But if the suggested evolu- 

 tional trend toward the simplification of ribs in Dinor- 

 this be correct, it would be desirable to continue the 

 usefulness of Pleeslomys by considering it as of sub- 

 generic value. On nomenclatorial grounds Dinorthis 



" Bull. 87, U. S. Geol. Surv., pp. 215, 308. 

 ^*Ann. Carnegie Mus., vol. 7, no. 2, p. 239. 

 ^° Zittel-Eastman Text-book of Paleontology, 2d ed., 

 vol. 1, p. 382. 



has priority over Pleeslomys because it appears first in 

 HaU and Clarke's book. Accordingly we suggest that 

 Dinorthis be the generic name under which the whole 

 assemblage of these related brachiopods be designated, 

 and to bring out the tribal relationships we propose that 

 this genus be divided into five subgenera as defined 

 on an earlier page. 



Subgenus MARIONELLA Bancroft 1928 



PI. 8, figs. 16-18 



Bancroft, Mem. and Proc. Manchester Lit. and Philos. Soc, 

 vol. 72, 1928, p. 181, pi. 2, figs. 13-16. 



Genoholotype. — M. typa Bancroft 1928. 



Description. Exterior. — Dinorthids of medium or 

 small size, subelliptical in outline; hinge-line straight; 

 cardinal extremities rounded. Lateral profile con- 

 vexo-plane to convexo-concave. Anterior commissure 

 faintly sulcate, sulcus shallow; ventral fold low, ob- 

 scure. Interareas short, the ventral one apsacline, the 

 dorsal orthocline. Ornamentation finely multicostellate. 



Ventral Interior. — Delthyrial cavity shallow; dental 

 plates strong, flaring, continued as ridges about the 

 periphery of the muscle field. Muscle area and pallial 

 markings as in Dinorthis. 



Dorsal interior. — Notothyrial platform shallow; 

 brachiophores of the Orthls type as developed in Dinor- 

 this ; cardinal process slender, simple, adductor field 

 small. 



Geologic range. — The single known species, M. 

 typa, comes from the Middle Ordovician of Wales. 



Distinguishing characters. — Marlonella differs 

 from Dinorthis s. s. and Pleeslomys only in its much 

 finer ornamentation and "the frequent, but not uni- 

 versal, presence of a narrow (sharply pinched up) 

 ventral mesial fold" (p. 181). 



Discussion. — The genus was first placed by its 

 nomenclator as a member of his subfamily Harkness- 

 ellinse, but this reference appears to us wrong. Ban- 

 croft in a letter tells us that he now believes Marlonella 

 to be a subgenus of Dinorthis. Specimens sent by him 

 to us are molds of the interior and exterior which give 

 no information regarding the structure of the shell, 

 whether punctate or impunctate. Our presumption is 

 in favor of an impunctate test because the other internal 

 features fit best with this type of shell, for example the 

 simple cardinal process. 



Characteristic dinorthid features occur in both valves 

 as follows: In the ventral, the musculature and pallial 

 markings are typical (see pi. 8, fig. 17). The car- 

 dinalia are of the orthoid type. In young forms the 

 crenulated myophore of the cardinal process is not 

 visible in the narrow slots of the internal molds. 



Marlonella is a parallel development to Pleeslomys, 

 evolved probably from the British Dinorthis. No shells 

 exactly like it are known in North America, and be- 

 cause of this independent development it deserves at 

 least subgeneric designation. 



