GENERA OF THE SUBORDER ORTHOIDEA 



67 



tire specimen apparently found in the drift and there- 

 fore without known locality and horizon. Verncuil, 

 upon the advice of Von Buch, who saw Schlotheim's 

 original specimen, states that it has five ribs in the 

 sulcus. This is not a distinctive character, as many 

 species share this peculiarity. Subsequent workers have 

 found great difficulty in identifying their localized shells 

 and in consequence have united different forms under 

 the same name. Because of the lack of formation 

 and locality of Schlotheim's type, McEwan suggested 

 that a different species be selected as the genotype. 

 She says (p. 388): 



As it is impossible to determine what Schlotheim's type- 

 specimen was it can not stand as the type of the genus. 

 Platystrofhia laticosta Meek is well known to all investiga- 

 tors of paleontology. As it possesses all the qualities neces- 

 sary for a genotype, it is suggested that future workers 

 regard it as such. 



This suggestion brought a protest from Bather^* 

 and a discussion of the genotype from Dietrich.'® The 

 latter defines P. bijorata (Schlotheim) as belonging to 

 the low-fold sub-group of the tricostate division, having 

 five costx in the sulcus and six on the fold, and nine 

 on the lateral slopes, as Von Buch had previously stated. 

 Its locality is either the north German diluvium or the 

 Baltic Ordovician. Dietrich's description follows: 



Das in Berlin befindliche Original der Platystrofhia 

 bijorata ist ein massig erhaltenes Schalcnexemplar von 22 

 mm. Breite, 16 mm. Hohe und 13.5 mm. Dicke. Es hat 

 auf dem Sinus 5 gleichstarke ungespaltene Rippen, auf dem 

 Wulst deren 6. Auf den Seitenflachen zahlt man in beidcn 

 Klappen 9 scharfe Rippen. Um die glatten Wirbel herum 

 verwischen sich alle Rippen; der Sinus ist dort seicht und 

 am Delthyrium macht er einer kleinen Aufwolbung Platz, 

 auf die die Rippen nicht hinaufgehen; diese erschcincn, 

 soviel zu sehen ist, alle gleichzeitig und keine ist bevorzugt. 

 Auf dem Wulst entwickeln sich die Rippen durch Teilung.^" 



This redescription of the genotype and its suggested 

 affinities with the tricostate, low-fold group of Platy- 

 strophias are the data that were needed to establish its 

 position. Cumings, after considerable study of the 

 American and European species, found that those from 

 Europe usually differed from their American relatives 

 in the costation pattern of the sulcus. On the basis of 



"Geol. Mag., vol. 57, 1920, pp. 88-90. 



>» Centralblatt f. Min., etc., 1922, pp. I23-12+. 



*° (Translation) The original specimen of Platystrofhia 

 bijorata, which is located in Berlin, is a fairly well pre- 

 served example about 22 mm. wide, 16 mm. long, and 

 13.5 mm. thick. In the sinus [sulcus] it has 5 equally 

 strong, undivided ribs, on the fold there are 6. On the 

 lateral slopes one counts 9 sharp ribs on both valves. About 

 the beak all of the ribs disappear; the sulcus in that place 

 is shallow and at the delthyrium it passes into a small swell- 

 ing, on which the ribs do not go; so far as can be seen, 

 these appear simultaneously and none is favored. On the 

 fold the ribs develop by division. 



this study he stated that the "presumptions arc strongly 

 in favor of its [the genotype of Platystrofhia^ being 

 biplicate [bicostate]" (p. 18), and accordingly he 

 restricted the term bijorata to bicostate forms. If 

 Dietrich's determination of the genotype as a tricostate 

 form be correct, it is evident that the bicostate species 

 of "bijorata," will need to be replaced by new specific 



names. 



Cumings' splendid monograph on the "Morpho- 

 genesis of Platystrofhia" gave the first careful analysis 

 of the genus. It was in this paper that the supposed 

 marked difference between the American and Euro- 

 pean Platystrophias was first indicated. These ideas 

 have been extended and elaborated by McEwan. Ac- 

 cording to Cumings, three divisions may be distin- 

 guished as follows: (1) uniplicate, (2) triplicate, and 

 (3) biplicate. Since in the present work the term 

 plication is restricted to a major undulation of the shell, 

 such as a fold or the undulations of the shell in 

 Enteletes, we here suggest that these terms be altered 

 to unicostate, tricostate, and bicostate. The three 

 groups may then be defined as follows: 



( 1 ) Unicostate grouf. — Here there is one costa in 

 the sulcus and two on the fold at the end of nepi- 

 onic development, and this condition continues then 

 throughout life. 



(2) Tricostate grouf. — In this group the primary 

 costa of the unicostate condition remains unmodified 

 throughout life, but in the early neanic stage a costa 

 is implanted on each slope of the sulcus. Simultane- 

 ously the two primary costae of the fold bifurcate. 



The tricostate group is merely a modification of the 

 unicostate one but it is convenient to continue the two 

 groups for their stratigraphic value. Under the tri- 

 costate group McEwan recognizes three subgroups: 

 (a) low-fold, (b) high-fold, and (c) Ponderosa. 



(3) Bicostate grouf. — Here the median costa of 

 the sulcus of the early neanic shell bifurcates, and 

 simultaneously a costa is implanted between the two 

 primary costa: of the fold. Four subgroups are recog- 

 nized by McEwan but a comprehensive study of the 

 European forms would doubtless demonstrate the need 

 for more subdivisions. 



Cumings has shown that the nepionic shell of 

 Platystrofhia has the sulcus in the dorsal fold and a 

 fold on the ventral. In the neanic stage, however, the 

 costs bounding the sulcus become elevated to form a 

 fold, and with this change the median rib of the dorsal 

 valve is depressed to form the lone costa of unicostate 

 shells. From this fundamental unicostate type the 

 tricostate and bicostate groups have evolved. 



According to McEwan and Cumings, divergence 

 of the tricostate and bicostate stocks must have taken 

 place in early Ordovician or Upper Cambrian time, 

 this because the modification of "the plications [costae] 

 of the fold and sinus [sulcus] takes place before the 

 shell has reached a length of 1 mm." The tricostate 

 group is thought to have diverged "from the ancestral 



