1 24 ( ht m mgs — Morphogenesis of PlatystropMa. 



the sinus and the beginning of another plication on either side 

 (i. e., the type is fundamentally triplicate). The neanic stage 

 of the Silurian forms (15, 16) has two plications in the sinus, 

 arising by bifurcation of the median plication of the nepionic 

 stage (i. e., the type is fundamentally biplicate). The early 

 ephebic stages of upper Ordovician forms correspond closely to 

 fully adult stages of lower and middle Ordovician forms (cf. 

 19 and 20 with III and IX; 22 with VI; 23, 24 and 25 with 

 VII). 



Distrihution of PlatystropMa. — On the map of the North- 

 ern Hemisphere (fig. 24) is shown the distribution of Platy- 

 strojihia* Most of these localities have already been men- 

 tioned. Those which have been added through recent investi- 

 gations may be mentioned briefly. 



PlatystropMa hiforata (?) has been reported from Baffin 

 Land by Schuchert.f The specimens are said to be small and 

 narrow at the hinge. Whiteaves cites the same species from 

 Akpatok Island, on the south side of Hudson Strait.:); 



Recently P. hiforata has been collected from the mainland 

 near Nova Zembla (Khabarova) by the Swedish North Polar 

 Expedition under Nansen.§ Johan Kieer, in a recent paper, 

 reports P. hiforata from near Christiania, Norway. || Weller 

 reports P. lynx from New Jersey, from a horizon low in the 

 Trenton. Tf White found the species in New Mexico at Silver 

 City.** Professor H. S. Williams has a specimen of P. lynx 

 from Arkansas. 



It will be noticed that the genus has never been found in 

 the Southern Hemisphere, nor in any region adjacent to the 

 Pacific Ocean. Nevertheless the area of its known distribu- 

 tion represents nearly forty degrees of latitude and one hun- 

 dred and seventy degrees of longitude. 



We should not be too hasty in drawing conclusions from the 

 above facts of distribution, in regard to questions of correlation 

 and Paleozoic geography; nevertheless several points seem to 

 me, if not demonstrated, at least forcibly suggested. 



The biplicate type of PlatystropMa, so characteristic of the 

 European Ordovician, is absolutely wanting in the American 

 Ordovician outside of the Hudson-Champlain trough. This 



* This map is based on a stereographic projection upon the plane of the 

 Equator, prepared by Professor S. L. Penfield. (See this Journal, vol. xiii, 

 May, 1902, p. 252.) The amount of distortion within the 40° of latitude is 

 slight. 



fProc. U. S. National Museum, xxii, 1900, pp. 151, 158. 



% Tuis Journal (4), vii, 1899, pp. 433, 434. 



k Johan Kiser, Sci. Pes. N. Polar Expd., vol. iv, xii, 1902, p. 7. 



|| Separataftryk af Norges Geologiske Undersogelses Aarbog for 1902, p. 61 



If Kummel and Weller, Bull. Geol. Soc. Amer., 12, 1901, p. 158. 



** Geol. Expl. West of the 100th Meridian, 1874, iv, p. 74. 



