Cumings — Morphogenesis of Platy strop hia. 17 



specimens leave us almost equally in the dark on this point.* 

 Fig. 12 of biforata certainly seems to be of the triplicate 

 type, the same as our Trenton and Cincinnati lynx. The var. 

 JZssieostata* on the other hand, is more probably of the bipli- 

 cate type; as are certainly all the Wenlock forms figured. 

 On the whole, it seems quite likely that the majority of 

 Russian, if not of all European Ordovician forms of Platy- 

 strophia, belong to the biplicate type. In this country, how- 

 ever, the biplicate type is absolutely restricted to the Silurian 

 and basal Trenton.^ 



This difference in the mode of origin of the plications of the 

 fold and sinus is of fundamental importance. It has been 

 shown that in nepionic stages of all forms, whether American 

 or European, Ordovician or Silurian, there is a median plica- 

 tion or fold on the ventral valve and a median sulcus or sinus 

 on the dorsal valve. This nepionic ventral fold becomes the 

 primary plication of the ventral sinus of later stages. In trip- 

 licate and uniplicate types this primary plication remains 

 simple ; but in biplicate types it bifurcates near its point 

 of origin, i. e., in an early neanic stage. Such a difference 

 characterizing so early a stage of the shell certainly points 

 to the origin of the peculiarity very early in the history 

 of the genus; and it is consequently very significant that 

 the only place where the two types, biplicate and triplicate, 

 are associated together in American faunas, is in the basal 

 Trenton of the Hudson-Champlain area.J The question 

 whether there is a similar association of the two forms in the 

 early Russian deposits cannot with the material at present 

 available be definitely settled. 



Platystrophia seems to have made its appearance in Russian 

 seas at fully as early an epoch as in American. Wysogorski 

 states that it occurs first in the Echinosphaerites limestone.§ 

 Schmidt lists P. lynx from Erras, Reval and Odensholm in 

 beds from the " vaginatum" limestone to the Borkholm.|| Wl. 



* Silurian Brachiopoda, 1871, pp. 268-273, pi. xxxviii, figs. 11-25. 



f These basal Trenton forms from the Hudson-Champlain area will be 

 described under the biforata type, to which they belong. The fact that they 

 are of the biplicate type, taken in connection with the further fact, brought 

 out by Kuedeinann, of the general European aspect of the Lower Ordovician 

 faunas of the Hudson Valley area, is certainly in striking accord with the 

 evidence already adduced as to the predominance of the biplicate type of 

 Platystrophia in European faunas. See also Ulrich and Schuchert, Bull. 

 N. Y. Mus. No. 52, 1902, pp. 633-663. 



% An apparent exception to this statement has been discussed under the 

 subject of neanic stages. 



§ Entwicklungsgeschichte, pp. 14, 15. 



|| Untersuchung iiber die Sil. Form, von Ehstland, etc. Archiv. fur die 

 Naturkunde Liv.-, Ehst.-und Kurlands, Iter ser., 2ter Band, 1858, p. 213. 



Am. Jour. Sci.— Fourth Series, Vol. XV, No. 85.— January, 1903. 



2 



