80 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



American Species 

 Ddtatreta elegantula Butts 1926 



Distinguishing characters. — ^The chief generic 

 characteristics of Paurorthis are the external dalmanel- 

 loid form combined with a ventral musculature inter- 

 mediate between that of Productorthis and Orthis, 

 while the cardinalia have the essential features of those 

 of Orthis. 



Discussion. — Paurorthis externally resembles cer- 

 tain members of the Dalmanellidas, as Wattsella and 

 Dalmanella, but is separated easily from these and all 

 other punctate orthids by its impunctate shell. The 

 punctures of the external shell layer of Paurorthis must 

 not be confused with the true punctation of the Dal- 

 manellidas, which never has any expression on the ex- 

 terior unless the shell has been abraded. Paurorthis 

 differs from Dalmanella further in its internal features 

 (compare figs. 5-7 of pi. 3 with figs. 2, 4, 31 of pi. 

 17). The ventral musculature is closest to that of 

 Productorthis in the elevated muscle area and the 

 arrangement of the scars. The cardinalia relate the 

 genus most closely to Orthis in the simplicity of the 

 brachiophores and the cardinal process. In Paurorthis 

 the latter is a simple ridge, but in all of the Dalman- 

 eUidse the cardinal process is always multilobate. Paur- 

 orthis is therefore clearly an Orthis that has converged 

 in external form to Dalmanella, and in this is an excel- 

 lent example of homoeomorphy. 



Paurorthis is also externally similar to Archceorthls 

 in the Dalmanella-Vkt form and contour but differs 

 internally in the following characters: In the ventral 

 valve of Paurorthis there is usually a fairly well defined 

 median ridge, expanded in front, which divides the 

 ovarian impressions. This, however, is not constant in 

 all specimens, some having a rather wide median undu- 

 lation. In ArchcBorthls, on the other hand, there is no 

 such ridge but the front of the muscle area is extended 

 forward as a lobe formed by adventitious shell. Still 

 more fundamental is the receding character of the 

 dental lamellae of Archworthls as compared with those 

 of Paurorthis, in which the front margins of the plates 

 extend vertically or obliquely forward to meet the 

 floor of the valve. Reniform ovarian impressions were 

 not seen in Archworthls. 



In the dorsal valve of Archworthls, the brachio- 

 phores are supported by convergent plates which unite 

 with a short median septum. In Paurorthis, on the 

 other hand, the brachiophores are orthoid in character 

 and supported by a deposit of adventitious shell in the 

 notothyrial cavity, and the median ridge is more prom- 

 inent and the muscle field wider than in Archeeorthls. 

 The exact taxonomic position of Orthis farva has 

 long been in doubt. In 1897 Schuchert placed it in 

 the genus Dalmanella. This reference was followed 

 by Walcott in the text of his monograph on the Cam- 

 brian brachiopods, but when the shell was figured on 



his plate 96 it was called Plectorthls. Raymond'* 

 places O. farva in Dalmanella and states that it "is in 

 reality exceedingly punctate," and Walcott figures a 

 thin section showing punctas (his fig. 10 on p. 99). 

 The junior author examined this section and found 

 that it is not of O. farva; it is of a true dalmanellid 

 of the D. rogata type, having endopunctas. In the 

 U. S. National Museum collections there are several 

 specimens of Dalmanella s. s. labelled D. farva but 

 none of them belongs to Paurorthis. 



Wysogorsky*" was the first to indicate that O. farva 

 is actually an impunctate shell, but did not recognize its 

 generic individuahty. Recently Kozlowski*^ deter- 

 mined the shell structure of the species but failed to 

 note the character of the punctae. The experience of 

 the present writers corroborates Wysogorsky's findings 

 that Paurorthis as here defined is impunctate but the 

 exterior of the shell is pitted. Thin sections show no 

 endopunctas. Moreover, the internal structure, mus- 

 culature, and cardinalia of Paurorthis are like those of 

 impunctate Orthis s. s., and have no resemblance to 

 similar structures in Dalmanella. 



Genus ARCH.ffiORTHIS Schuchert and 

 Cooper 1931 



PI. 6, figs. 7, 8, 16 



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

 p. 243. 



Genoholotype. — Orthis electra Bilhngs 1862, 

 Pal. Foss., vol. 1, p. 79, fig. 72; p. 217. 



Description. Exterior. — Shell small, dalmanel- 

 loid; anterior and lateral margins rounded; cardinal 

 extremities obtuse ; hinge-line narrower than the great- 

 est shell width ; lateral profile unequally biconvex, the 

 ventral valve usually having the greater convexity; 

 lateral commissure straight; anterior profile unequally 

 biconvex; anterior commissure faintly sulcate. A faint 

 sulcus developed in the dorsal valve. Ventral interarea 

 longer than the dorsal, apsacline, curved, beak slightly 

 incurved, umbo convex; dorsal interarea short, ana- 

 cline, umbo swollen. Delthyrium open or closed by 

 a deltidium. Ornamentation multicostellate to fasci- 

 costellate. Shell structure unknown. 



Ventral interior. — Delthyrial cavity deep; teeth 

 strong ; crural fossettes very deep ; dental plates thick, 

 receding, obsolete in old shells, forming the walls of the 

 delthyrial cavity, but the delthyrial margin is formed 

 by the strongly overhanging edges of the palintrope. 

 The dental plates in young shells are continued for- 

 ward as a low ridge about the muscle area. The latter 

 is on a callosity that extends from the front of the 

 delthyrial cavity nearly to the middle of the shell. 



="• Bull. Mus. Comp. Zool., vol. 66, no. 3, 1916, p. 265. 

 ^^Zeits. deut. geol. GeseU., vol. 52, pt. 2, 1900, p. 234. 

 " Pal. Polonica, vol. 1, 1929, p. 53, footnote. 



