GENERA OF THE SUBORDER ORTHOIDEA 



149 



EnieUtes. In other words, this is essentially an 

 EnteUtes showing atavistic tendencies toward Ortho- 

 Uchia in the loss of the radial plications and globose 

 profile. 



Discussion. — The absence of interareas, while not 

 impossible, is not convincing from the figures presented 

 by Stuckenberg, especially figure 8e of plate 6. Such 

 an arrangement of the dental plates and their connec- 

 tion with the lateral margins of the shell as indicated 

 suggests a wide hinge-line, and the latter presupposes 

 a palintrope. The shells of Orthotichia and Enteletes 

 are usually thin and very susceptible to crushing; 

 this may be the reason for the apparent lack of inter- 

 areas in Stuckenberg's specimens. Parallelism of the 

 dental plates would be an important clue to deter- 

 mining the ancestry of this form, but the figures of 

 the author are not specially convincing on this point 

 (see his fig. 8b, pi. 9). It may be questioned whether 

 an Enteletes that has atavistically lost its radial orna- 

 mentation would likewise lose its most characteristic 

 internal features and the globose contour of the shell 

 as well. 



Schellwien has made much of the supposition that 

 many species now referred to Orthotichia may actually 

 represent atavistic Enteletes and for this reason he 

 referred all smooth forms having the internal struc- 

 ture of Enteletes to that genus, placing there even the 

 genotype of Orthotichia. It is important, if it can be 

 proved that atavistic forms of Enteletes actually exist, 

 that these be separated under a new designation and 

 not included in Orthotichia, which is essentially an 

 incipient or potential Enteletes. It is also important to 

 determine when these atavistic tendencies appear. 

 They may have appeared soon after the origin of the 

 genus, occurring several times during the Upper Penn- 

 sylvanian and Permian, or they may have been delayed 

 till near the end of the Permian, in which case the 

 matter is more simple. King (1931) maintains that 

 the Permian Enteletes, in large part, are more strongly 

 plicated than the Pennsylvanian species, and that the 

 forms with faint plications do not even resemble Ortho- 

 tichia in profile. If the atavism is actually carried to 

 the degree of the resumption of the convexo-concave 

 form of Orthotichia, the only reliable clue to the rela- 

 tionship of atavistic shells would be in the parallelism 

 of the septal plates of the ventral valve. The paral- 

 lelism of the plates in Enteletes is frequently more 

 apparent than real, and it is often difficult to evaluate 

 the degree of divergence that does exist. It will thus 

 be seen that it is extremely difficult to establish the 

 fact of atavism in these shells. It seems to the writers 

 that unless the atavistic tendencies are inaugurated in 

 the nepionic stage of Enteletes, Enteletoides should 

 have the globose outline and interior of the former 

 genus. If the return of the previous characters takes 

 place in the nepionic stage, all objections in regard to 

 profile in internal characters noted above lose their 

 force. 



Another species referred here is Enteletoides sub- 

 rossicus Gorsky 1927, from the Middle Carboniferous 

 on Kamenka River, eastern Urals. 



Subfamily ISORTHIN^ Schuchert and Cooper 

 1931 



Divergent biconvex Schizophoriidae, having the ven- 

 tral musculature, cardinalia, and pallial markings of 

 Schizophoria. The only known genus is Isorthis 

 Kozlowski, probably derived out of early Silurian 

 Schizofhoria. 



Genus ISORTHIS Kozlowski 1929 



PI. 21, figs. 17-28, 30-33; pi. 23, figs. 15, 19; 

 t. fig. 13 



Kozlowski, Pal. Polonica, vol. 1, 1929, pp. 29, 75, pi. 2, 

 figs. 24-41, t. figs. 16-18. 



Genoholotype. — Dalmanella {Isorthis) szajno- 

 chai Kozlowski 1929. 



Description. Exterior. — Transversely subellipti- 

 cal, hinge-line straight, narrower than the greatest 

 width of the shell; cardinal extremities rounded. 

 Lateral profile unequally to subequally biconvex; an- 

 terior commissure rectimarginate to faintly sulcate. 

 Dorsal sulcus shallow. Ventral palintrope longer than 

 the dorsal, curved, gently apsacline. Beak curved, 

 umbo swollen; delthyrium open. Dorsal palintrope 

 short, faintly apsacline or faintly anacline ; notothyrium 

 closed by the cardinal process. Ornamentation multi- 

 costellate. Shell fibrous, punctate. 



Ventral interior. — Delthyrial cavity deep ; teeth 

 large; crural fossettes deep; dental plates strong in 

 young specimens, obsolete in old ones. Muscle field 

 deeply impressed, bilobed in front ; diductor scars elon- 

 gate, subparallel; adductor track elevated, usually nar- 

 row, adductor scars semielliptical when visible ; adjus- 

 tor scars located on the sides of the dental plates, 

 usually not clearly visible. Pallial markings promi- 

 nent, consisting of two main trunks extending antero- 

 laterally from the anterior end of each diductor. 



Dorsal interior. — Brachiophores as in Schizophoria, 

 i. e., bladelike plates, with fulcral plates and usually 

 supported by adventitious shell (see below). Cardinal 

 process small, bilobed, trilobed, or multilobed, com- 

 monly modified by deposition of adventitious testaceous 

 deposit. Median ridge low. Muscle area subcircular 

 in outline, usually with thickened peripheries. Adductor 

 scars subequal in size. Pallial sinuses consisting of three 

 pairs of trunks as in Schizophoria; two originate at 

 the antero-medial ends of the anterior adductor scars 

 and extend anteriorly; a second set starts just outside 

 the former and extends antero-laterally. The third 



