l s Cumings — Morphogenesis of Platystrophia. 



Lamansky cites Platystrophia oiforatairom the " planilimbata" 

 limestone.* As to the correlation of these beds with American 

 strata there is unfortunately no unanimity of opinion. Schmidt 

 La disposed to consider the Echinosphserites limestone as the 

 equivalent of the Quebec Group of this country; and the 

 vaginatum as equivalent to our Black River formation. f In 

 Kayser's Lehrbuch der Geologie (1891) the Echinosphserites 

 limestone is placed higher than the Trenton and the vagina- 

 tum as equivalent to the Trenton.J The planilimbata lime- 

 stone would then be about equivalent to our Black River. Sub- 

 stantially the same correlation is given by Neumayrg and by 

 de Lapparent.il It seems probable, therefore, that the first 

 appearance of Platystrophia in Europe was at least as early as 

 in this country, and perhaps earlier : and since it was during 

 this early Ordovician epoch that the two main types of the 

 genus were being differentiated, we may expect to find both 

 in the European as well as in the American deposits of that 

 age. 



From the above discussion it appears that there existed after 

 the early Ordovician two distinct types of Platystrophia, one 

 of which (the triplicate) characterized the American Ordovician, 

 and the other (the biplicate) the European Ordovician and the 

 Silurian of both America and Europe. Both of these types or 

 species include several varieties. It is also evident that the 

 terms lynx and biforata have been very loosely applied to both 

 of these types. Since the term lynx has become so strongly 

 associated with the American group of shells (triplicate type), 

 I propose to limit the term to such American Ordovician shells. 

 It is impossible to say whether the type of P. oiforata is 

 biplicate or triplicate. The presumptions are strongly in 

 favor of its being biplicate. I, therefore, propose to restrict 

 the term to biplicate forms such as the Wesenberg specimen 

 above described. It will then include Ordovician forms of 

 Europe, and Clinton forms of both Europe and America. For 

 the biplicate shells with few plications such as are found in 

 Gotland, the Wenlock of England, and Anticosti, the term 

 dentata will be used. Further details of the species biforata 

 and dentata will be given later. 



Derivation of Platystrophia lynx. — According to the above 

 interpretation and restriction of P. lynx, this form probably 



* Neue Beitrage zur vergleichung des Ost-Baltischen und Scandinavischen 

 Unter-Silurs. Centralblatt fur Min. Geol. und Pal., Neues Jahrb., 1901, No. 

 20, pp. 611-618. 



f Quar. Jour. Geol. Soc, xxxviii, 1882, pp. 520, 521 ; Archiv. Liv.-, Ehst.- 

 und Kurlands, Iter ser., 2ter Bd., 1858, p. 48. 



J Op. cit., p. 64. § Erdgeschichte, 1895, vol. ii, p. 98. 



|| Traite de Geologie, 1900. p. 824. 



