136 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



character of the teeth are other important anatomical 

 features of the genus. 



Discussion. — The members of Platyorthis have 

 been variously referred to Dalmanella and Rhipuiotn- 

 ella, but the combination of characters, both internal 

 and external, makes this genus unique and distinguishes 

 it from all other members of the Rhipidomellidae or 

 Dalmanellidas. The cardinal process is notable for its 

 extremely large size, a development that is probably 

 the result of the flattening of the dorsal valve. The 

 internal characters of Platyorthis ally it most strongly 

 to RhifUomella, and the group in all probability repre- 

 sents a deviation from the main Rhifidomella line. In 

 Europe it appears in the Borszczow stage of the Silu- 

 rian, occurs in the Siegener Schichten of the low^er 

 Coblenzian, and extends into the Middle Devonian 

 (Eifelian). In America it is of Devonian time and 

 ranges from the Becraft through the Oriskany and into 

 the Grande Greve limestone of Quebec, which is lower 

 Middle Devonian in age. 



Genus THIEMELLA Williams 1908 

 PI. 19, figs. 1, 2, 4-6, 9 



Williams, Proc. U. S. Nat. Mus., vol. 34, 1908, p. 59, 

 pi. 2, figs. 11, 12, 16, 19, 22, 23. 



Genoholotype. — T. vlllenovla Williams 1908. 



Description. Exterior. — Like Rhifidomella ex- 

 cept for the presence of a well marked ventral fold 

 and an equally prominent dorsal sulcus. In young 

 stages the fold and sulcus are reversed, and in mature 

 and old forms both may become obsolete at the front. 



Ventral interior. — Delthyrial cavity shallow, dental 

 plates low, continued as a low ridge around the pe- 

 riphery of the muscle area, obsolete in old shells. Mus- 

 cle field suboval to subcordate; diductor impressions 

 subcrescentic, somewhat expanded in front; adjuster 

 scars narrow. Adductor field an elliptical impression 

 imprisoned by the diductor scars. Extending forward 

 from the front end of the adductor field to beyond the 

 middle of the valve is a low ridge. Lateral spaces 

 coarsely pustulose. 



Dorsal interior. — Cardinalia abbreviated and deli- 

 cate; brachiophores as in Rhifidomella, divergent 

 plates attached to the inner surface of the valve. 

 Cardinal process with a short shaft, and the myophore 

 a lobate boss on the notothyrial platform; median 

 ridge low and obscure in adults. Adductor field 

 confined. 



Geologic range. — Upper Devonian of eastern 

 United States. 



Species 



Thiemella villenovia Williams 1908 

 Dalmanella tenuilineata (Hall) 18+3 



? Dalmanella danbyi Williams 1908 

 ? Orthis leonensis Hall 1 867 



Distinguishing characters. — Thiemella com- 

 bines some of the generic features of Schizofhoria, 

 Dalmanella, and Rhifidomella. The external form is 

 essentially that of Rhifidomella, but add to this a 

 prominent dalmanelloid fold and sulcus, and the ex- 

 terior is unlike that of any other dalmanellid except 

 Aulacella. Williams named as one of the generic 

 characters the reversion of the fold and sulcus from a 

 ventral sulcus and dorsal fold in the young stages to 

 a ventral fold and dorsal sulcus in mature forms. In 

 many specimens, however, the fold and sulcus are lost 

 at the front of the shell. 



Discussion. — Thiemella is closest to Aulacella of 

 the Dalmanellidas from the Eifelian of the Rhine. In 

 this form the fold and sulcus are not reversed and 

 internally there are other important diflFerences from 

 Thiemella. In the ventral valve of the latter the 

 median ridge extending forward from the front end 

 of the adductor field is undivided and continuous to 

 beyond the middle of the shell. In the German genus, 

 on the other hand, the low ridge dividing the diduc- 

 tor scars at the front is forked near the front margin 

 of the diductor impressions. According to Williams, 

 the "forked septum" is a feature characteristic of Dal- 

 manella but in our experience it has proved not to have 

 marked generic value. 



In the dorsal valve the cardinalia of Thiemella are 

 remarkable for their delicacy, whereas Aulacella has 

 ponderous structures for so small a shell. These differ- 

 ences, when combined with the geologic age and geo- 

 graphic distribution, help to distinguish the two genera. 



Family HETERORTHID^ Schuchert and 

 Cooper 1931 



Broad, flat or concavo-convex, divergent Dalman- 

 ellacea, having subflabellate ventral diductor impres- 

 sions, not enclosing the adductor marks. Brachio- 

 phores like those of Dalmanella, cardinal process nar- 

 row, with an elongate myophore. In the cementation 

 of the brachiophores to the floor of the valve, tissue is 

 deposited under their dorsal surface and is not uncom- 

 monly continued anterior to the front ends of the 

 brachiophores. This mode of cementation is not unlike 

 that of Rhifidomella and suggests that the heteror- 

 thids may have arisen from the same stock of dalman- 

 ellids that gave rise to Rhifidomella. 



The family is divided into the subfamilies Heteror- 

 thinas Schuchert and Cooper and Harknessellinje Ban- 

 croft. The genetic relations are apparently as shown 

 in Table 12. 



