16 ( '"in ings — Morphog< nesis of Platystrophia. 



appear at the apex ; between these, one is implanted and almost 

 immediately bifurcates; and between these two, another is 

 implanted and bifurcates, making six in all. 



Another specimen of P. hiforata from Wesenberg, Russia, 

 has four plications in the sinus and five on the fold, but they 

 originate in precisely the same way. The last plication added 

 on the fold does not, however, bifurcate ; and the unpaired 

 median plication of the sinus is wanting. 



In a specimen from Pulkowa, Russia (near St. Petersburg), 

 labeled P. lynx, there are (tig. 6) six plications on the fold and 

 five in the sinus. These originate as follows : In the sinus one 

 plication appears at the apex and immediately bifurcates. Each 

 of the two thus formed bifurcate, and the one to the left bifur- 

 cates again, giving five in all. On the fold two plications 

 appear at the apex and one more is almost immediately 

 implanted between them. Each of these three now bifurcate, 

 giving, at the front margin, six plications. The specimen there- 

 fore belongs to the same type (biplicate) as the Wesenberg 

 specimens. It is impossible to make sure from the literature 

 of the Russian Platystrophia whether this peculiarity char- 

 acterizes all Russian forms. According to Wysogorski,* who 

 is very familiar with the Orthidce of the Baltic-Province, " the* 

 form from the Upper Silurian of Gotland is characterized by 

 smaller size [than the Russian form] while in other respects it 

 agrees with Platystrophia hiforata Schloth." 1 have examined 

 some thirty specimens of the Gotland form and these invari- 

 ably have three plications on the fold and two in the sinus. If 

 Wysogorski has taken this peculiarity into consideration, his 

 statement is strong evidence, in support of that afforded by 

 the Russian specimens which I have examined, as to the bipli- 

 cate character of all these Russian types. None of the earlier 

 authors shed any light on this point. De Verneuil gives 

 several excellent figures of the lynx, dentata and chama 

 (=costata) types and mentions a lynx from Grodno with two 

 plications in the sinus. In his figures of lynx four plications 

 are shown on the fold but their mode of origin cannot be made 

 out, though they are all shown to originate closer to the apex 

 than is common in our American lynx, and even closer than in 

 the lynx from Cincinnati, Ohio, figured on the same plate. 

 Whenever four plications are present on the fold of biplicate 

 types, they certainly come in closer to the apex than the four 

 plications of triplicate types. Nevertheless, such evidence as 

 afforded by the figures in question cannot be considered as of 

 any particular value. 



Davidson's figures and descriptions of British Ordovician 



* Entwicklungsgeschiclite, p. 15. 



