66 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



American Species — Cont. 



Ponderosa subgroup 



P. -frefonierosa McEwan 1919 

 fonderosa Foerste 1909 

 fonderosa auburnensis Foerste 1 909 

 fonderosa arnheimensis McEwan 1919 



High-fold subgroup 



P. frofundosulcata (Meek) 1873 



frojundosulcata hofensh Foerste 1910 



crassa (James) 1874 



laticosta (Meek) 1873 



unicostata Cumings 1903 



unicostata crassiformis McEwan 1919 



cyfAa (Jixnes) 1874 



cyfha tumida McEwan 1919 



cyfha arcta McEwan 1919 



cyf ha bellatula McEwSin 1919 



wallozvayi Foerste 1912 



European Species 



Unicostate group 



Porambonites costata Pander 



Sfirijer chatna Verneuil non Von Buch (Geol. Russle, 

 pt. 2, pi. 5, fig. 1) 



Tricostate group 



Terebratulites iiforatus Schlotheim 



Plaiystrofhta sublimis Opilc 



P. bijorata Reed (non Schlotheim) (Girvan Dist., p. 



845) 

 Sfirijer aferturata Schlotheim (Eichw. reports from 



Orthoceratites Is.) 

 Terebratula tenuicostata Eichwald 

 ? T. lynx Eichwald 

 ? Platystrofhia bijorata sardoa Vinassa de Regny 



Bicostate group 



Porambonites dentata Pander 



Orthis bijorata Davidson non Schlotheim (Brit. Foss. 



Brach. vol. 3, pt. 7, Sil., pi. 38, figs. 1 la, b) 

 Sfirijer tridens McCoy (may ^ bijorata') 

 Sfirijera bijorata fissicostata McCoy^° 

 Atryfa dorsata Hisinger^° (some of the forms placed 



here belong in Oxoflecia) 

 Platystrofhia "bijorata" — Silurian of Gotland 



Discussion. — Platystrofhia is unique among the 

 Orthidas. At first referred to Terebratulites, Tere- 

 bratula, Sfirijer, Delthyr'ts, etc., Davidson in 1848^^ 

 was the first to show that the internal characters of 

 the shell were clearly orthoid. In 1850 King pro- 

 posed the name Platystrofhia for the- group, basing it 

 on Terebratulites hijoratus Schlotheim. However, 

 the proposal of the new generic term for this group 

 was too radical a departure for the times. Hence one 



^' Orthis bijorata var. fssicostata Davidson non McCoy 

 (Brit. Foss. Brach., vol. 3, pt. 7, Sil., pi. 38, figs. 15-17) 

 may be an Oxoflecia. 



^® Platystrofhia dorsata Reed (Girvan Dist., pi. 8, fig. 

 25) non Hisinger is an Oxoflecia. 



" Bull. Soc. Geol. France (2), vol. 5, p. 323. 



finds later on that these shells are referred to Orthis 

 by Davidson. Eichwald, on the other hand, accepted 

 the name and by the early eighties it was in general 

 use. 



The most unique external features of this genus are 

 its spiriferoid form, best developed in P. acutilirata, and 

 the nearly equal length of the interareas. There are, 

 however, several species in which the hinge-line, in- 

 stead of being wide and alate, is actually narrowed and 

 the whole form of the shell is suggestive of Ortho- 

 rhynchula and the rhynchonellids. Such a form is 

 P. globata Twenhofel. It is by no means usual in 

 the genus that the interareas are equal; the ventral 

 palintrope is as a rule the longer. In some species the 

 ventral interarea is curved and the dorsal beak is 

 incurved exactly as in Sfirijer. 



The interior of Platystrofhia links the genus inti- 

 mately to Plectorthis and Hebertella, as best seen in 

 young specimens (see pi. 12, figs. 16, 19). The 

 ventral dental plates are discrete, continued directly 

 to the floor of the valve and marked off by deep um- 

 bonal cavities. In the dorsal valve the brachiophores 

 and their supporting plates are not divisible. The com- 

 bined plates converge to form a sessile cruralium. The 

 sockets are defined by a fulcral plate (see pi. 12, fig. 

 19). The only notable departure from the type of 

 structure usual in Plectorthis is in the cardinal process. 

 This is a low septum located on the middle of the 

 notothyrial platform. It is so low that it could never 

 have served as the seat of attachment of the diductor 

 muscles. It is possible that this septum or process is a 

 secondary development. Scars of diductor attachments 

 on the notothyrial platform can be seen commonly in 

 mature specimens. It is likely, too, that the impres- 

 sions or grooves on the inside face of the brachiophores 

 are in reality also impressions of the diductor muscles. 



In old shells a riotous deposition of adventitious shell 

 covers over all of the structures usually relied on in 

 taxonomies. The umbonal cavities laterad to the den- 

 tal plates and brachiophore supports are completely 

 filled. This obliteration of essential characters by 

 adventitious shell has caused the taxonomic relationships 

 of Platystrofhia to remain in obscurity for many years. 

 Barring the fold and sulcus, a young Platystrofhia and 

 a mature Plectorthis are almost identical internally and 

 externally. Young Platystrophias are strongly costate 

 internally, exactly as in Plectorthis, but in old age all 

 of this resemblance is lost. The microscopic structure 

 of the shell is like that of the Plectorthidas in being 

 fibrous and impunctate. Mrs. McEwan states that 

 "the inner surface is finely punctate"; nevertheless, 

 thin slices of the shell prepared by the writers show no 

 punctas. McEwan probably refers to the megascopic 

 pits which indent the inner surface of old shells, but 

 these have no relation whatsoever to punctas, and are 

 a feature of many orthoid shells. 



The genotype of Platystrofhia selected by King is 

 Schlotheim's Terebratulites hijoratus, based on an en- 



