GENERA OF THE SUBORDER ORTHOIDEA 



69 



when the nepionic shell of PUctorthis is discovered it will 

 be found to be quite indistinguishable from the nepionic 

 shell of Pljtsstrof<hui, since the two groups present at the 

 beaks almost identical characters. . . Pleclorthis may there- 

 fore represent an offshoot from the Platystrophia group 

 near its initiation or, . . . it may have been separately 

 derived from the Orlhis \Oruii<3\ Utituidaris stocic. 



The writers of the present work have gone even 

 farther than Cumings and show that the internal 

 characteristics of Plrctorthis and Platystrophm are al- 

 most identical. These facts link the two genera inti- 

 mately, but it is impossible at present to say if the one 

 was derived from the other. We are suggesting the 

 origin of the Plectorthid.i; in general from the Ozar- 

 kian F'mkelnhurgui, which has all the internal char- 

 acteristics of PUctorthis and Platystrophia. 



VVysogorsky"'* looked to Orthis calligramma as the 

 ancestor of Platystrophia, but his view is not tenable 

 since that species does not have internal characters from 

 which those of the genus under consideration could be 

 derived. The development of a strong fold and sulcus 

 is not the whole story in the evolution of this genus. 

 McEwan states (p. 388) that "Eoorthis of the Upper 

 Cambrian has the physiognomy of this genus and has 

 been confused with it." It must be emphasized, how- 

 ever, that the Orthids and Billingsellidas with their 

 peculiar t)'pe of divergent brachiophores could never 

 have evolved Platystrophia. 



Opik'* gives the origin of Platystrophia from 

 "Orthis" recta (Pander) as follows: 



Der Ursprung von Platystrophia ist in der Gruppe der 

 "Orthis" recta (Pander) aus B13 and Bna des Ostbaltikums 

 zu suchen. Unwahrscheinlicher ist die Ableitung aus 

 O. calligramma oder Orusia.^" 



"Orthis" recta is the type of our new genus Angusti- 

 cardinia ( 193 1 ). Internally it has features that might 

 lead to plectorthoid structures. The brachiophore sup- 

 ports are convergent but meet a short median ridge. 

 The specimens at hand are too poorly preserved to 

 show socket plates but such may have existed. Exter- 

 nally, however, Angusticardiriia has none of the requi- 

 site features for the Platystrophia ancestor. There is 

 a more or less well defined sulcus on each valve, the 

 one of the dorsal having as many as three costse. It 

 seems to be rather a plastic form, diverging toward 

 the rhynchonellids. Its only truly orthoid features are 

 the two interareas with their open delthyria. Inter- 

 nally the dorsal valve is not unlike the early rhyn- 

 chonellid species C amarotaechia ? plena (Hall) and 

 C. ? orientalis (Billings). Accordingly we hold that 



"Zeit. deut. geol. GeseU., vol. 52, 1900, p. 234. 



" Acta et Comment. Univ. Tartuensis, A, vol. 1 7, pt. 

 I, 1930, p. 102. 



'° (Translation) The origin of Platystrophia is to be 

 sought in the group of "Orthis" recta (Pander) from B13 

 and Biio of the east Baltic region. Less probable is the 

 derivation out of O. calligramma or Orusia. 



Angusticardinia is not the ancestor of Platystrophia but 

 probably a divergent stock of the Orthida; toward the 

 rhynchonellids. 



It is therefore impossible as yet to indicate the place 

 of origin for Platystrophia, but it seems more probable 

 that it was western Europe than northeastern North 

 America. On the other hand, there may have been 

 two origins, the American forms arising out of the 

 stock that gave rise to Plectorthis and the European 

 forms independently out of some unknown stock. 



Genus MCEWANELLA Foerste 1920 



PI. 12, figs. 11, 12, 17, 18,21,22 



Foerste, Bull. Sci. Lab. Denison Univ., vol. 19, 1920, p. 

 197. 



Genolectotype (Schuchert and LeVene 1929). 

 — Platystrophia fernvalensis McEwan 1919, Proc. 

 U. S. Nat. Mus., vol. 56, p. 428, pi. 50, figs. 1-3 = 

 Hebertella lineolata Savage 1918, Trans. Illinois Acad. 

 Sci., vol. 10, p. 267, pi. 1, figs. I, 2. 



Description. Exterior. — Platystrophiinae with the 

 whole surface multicostellate, the costells covering the 

 inherited Platystrophia costs or constituting all of the 

 ornamentation. 



Interior. — The ventral musculature and dorsal car- 

 dinalia are exactly like those of Platystrophia, but in 

 mature shells there appears to be very little deposition 

 of adventitious shell. 



Geologic range. — Upper Ordovician (Rich- 

 mond). 



American Species 



Hebertella lineolata Savage 1918 (syn. M. fernvalensis 



McEwan 1919) 

 Mcetvanella raymondi Foerste 1920 



Discussion. — Mcewanella was separated from 

 Platystrophia chiefly because of its peculiar ornamen- 

 tation. At the beaks the shell is usually costate as in 

 Platystrophia, but in all later growth the costs are 

 completely altered by bifurcation into costells. In 

 some specimens of M. lineolata the primary costas may 

 be lost at the front of the shell and the whole sur- 

 face is then occupied by costelL-c. This is a radical 

 departure from the costate and granulose surface of 

 Platystrophia. The surface of Mcewanella is a con- 

 vergence toward the type of ornamentation usual in 

 Eoorthis, and so well shown by E. ochus VValcott from 

 the Ozarkian. In M. raymondi the primary plica- 

 tions are strongly developed over the whole surface 

 of the shell. 



Internally, Mcewanella is clearly a Platystrophia. 

 The ventral muscle field has a similar thickened ridge 

 and the individual impressions have the same indistinct- 

 ness. The cardinalia have rather sharply pointed, 

 elongate brachiophores which have the supporting 



