68 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



stock much later than the biplicate [bicostate] group" 

 as the uniph'cate condition remains till the shell has 

 reached a length of 1.2 to 1.5 mm. (p. 391). 



Platystrophia first appears in the European Middle 

 Ordovician (Kuckers formation, Brandschiefer) in well 

 organized bicostate and tricostate species, P. dentata 

 and P. lynx. In the Schuchert Collection is a lot of 

 P. dentata labelled as "probably Echinosphjerites lime- 

 stone." If this horizon be correct, it pushes the first 

 appearance down into lower Middle Ordovician or 

 about the time of the Black River in America. These 

 Echinosphasrites limestone specimens are bicostate forms 

 and would not afford notable support to McEwan's 

 theory concerning the origin of the group. The 

 Kuckers may be upper Black River in age but is more 

 probably lower Trenton. According to McEwan 

 (p. 400), "a uniplicate species was found to occur 

 in the Jewe [Trenton], and one specimen was 

 found which occurred in the Upper Ordovician (F^) 

 [Lyckholm or uppermost Trenton]." The unicostate 

 group therefore appears to be a rather insignificant 

 assemblage. Both Cumings and McEwan maintain 

 that the bicostate group is the dominant one in Europe. 

 The foreign species are so poorly known that it is as 

 yet difficult to know what is the dominant group in 

 the way of species, but it is a fact that after their origin 

 the bicostate group held the European ground occupied 

 from the Middle Ordovician into the Middle Silurian. 

 In North America, however, the situation is just the 

 reverse, the tricostate group holding its ground through- 

 out the Middle Ordovician and into the Middle 

 Silurian. 



McEwan's table (pp. 402-404) indicates the first 

 appearance of Platystrophia in North America in Black 

 River time (Decorah shale of Wisconsin). Hall and 

 Clarke,-^ Winchell and Schuchert,^^ Ruedemann,^^ 

 and Schuchert^* all have reported Platystrophias from 

 the Chazy but neither McEwan nor Cumings^^ could 

 find any evidence to substantiate these reports. There 

 may also be some doubt as to the precise age of the 

 form P. extensa McEwan, said to have come from the 

 Decorah shale. This formation has recently been sub- 

 jected to a critical survey by G. Marshall Kay,"^ who 

 comes to the conclusion that the upper or Ion member 

 of the Decorah is actually lower Trenton in age. Kay 

 reports P. extensa McEwan from this member (upper 

 Ion, Prasopora faunule. Church, Iowa) in company 

 with P. trentonensis McEwan. This is the only ho- 

 rizon from which he has Platystrophia and it is there- 

 fore probable that the specimen from the Decorah of 

 Wisconsin is from the Ion member and actually lower 



" Pal. N. Y., vol. 8, pt. 1, 1892, p. 202. 

 " Geo!. Minn., vol. 3, pt. 1, 1895, p. 456. 

 ^^N. Y. State Mus., Bull. 49, 1902, p. 25. 

 " U. S. Geol. Surv., Bull. 87, 1897, p. 309. 

 ^''Amer. Jour. Sci. (4), vol. 15, 1903, p. S. 

 ^*Jour. Geol., vol. 37, 1929, pp. 639-671. 



Trenton in age. This would be in agreement with the 

 remarks made above on the European section. 



In North America the first Platystrophias to appear, 

 barring the one mentioned above, are bicostate forms 

 in the lower Trenton and one unicostate species, P. 

 uniplicata McEwan. If the so called Black River 

 form be considered lower Trenton in age, then the 

 three groups appear in North America almost simul- 

 taneously and fully standardized. The unicostate 

 group is represented here by one known species only. 

 After lower Trenton time the bicostate group is, so 

 far, unknown in the American Ordovician until the 

 topmost formation, the Gamachian (Ellis Bay) of 

 Anticosti, is reached. From this time on the bicostate 

 group is dominant in American Silurian strata and a 

 very few rare tricostate individuals are to be found in 

 the early Silurian. 



A recent table of suggested correlations by Ulrich^^ 

 places the Wierland group, of which the Echino- 

 sphasrites limestone and Kuckers are a part, as upper 

 Chazy in age. This would make the European species 

 antedate the American forms and would be rather sug- 

 gestive, although not final evidence, of a European 

 origin of the genus. The important point, however, 

 is not the precise age of their appearance but the fact 

 that, when Platystrophias are present for the first time, 

 the three groups come together, fully organized. 



The appearance of Platystrophia in early Trenton, 

 Black River, or late Chazy time is suggestive that the 

 ancestor of the group should be sought in rocks of 

 Chazy age. McEwan has prophesied a unicostate pro- 

 genitor, from which the bicostate group diverged first, 

 followed by the tricostate group. 



Both Cumings and McEwan have turned to the 

 Upper Cambrian faunas as being the possible source of 

 Platystrophia. Cumings found in Orusia lenticularis 

 (Wahlenberg) "a form that possesses in the adult 

 practically all of the nepionic characters of Platystro- 

 phia" (p. 5). Taking only external characters, this 

 form could have evolved the exterior of Platystrophia, 

 and internally it has subparallel, discrete brachiophore 

 plates, not unlike those of Platystrophia. It is possible, 

 of course, that crowding of the bases of these toward 

 each other could produce the condition seen in Platy- 

 strophia, but the early appearance of Orusia and its 

 short stratigraphic range make it unlikely that it is the 

 direct ancestor, although it may have been in the 

 general line. 



Cumings has also pointed out the external similari- 

 ties between Platystrophia and Plectorthis, as follows 

 (pp. 11-12): 



In some respects the adult Plectorthis plicatella resem- 

 bles the neanic Platystrophia. If the fold and sinus be 

 disregarded (and in some Trenton forms of Platystrophia 

 these are surprisingly inconspicuous), the neanic Platy- 

 strophia is almost a Plectorthis. There is little doubt that 



" U. S. Nat. Mus., Proc., vol. 76, 1930, p. 73. 



