46 ( '// m ings — Morphogejiesis of Platystrophia. 



Lawrence.* By another route the form reached the Missis- 

 sippi area of this country, where it is very poorly represented.! 



24 



Fig. 24. — Map showing distribution of Platystrophia. 1, Khabarova ; 

 2, Russian Province ; 3, Island of Gotland ; 4, West Gotland ; 5, Scot- 

 land ; 6, Wales ; 7, Ireland ; 8, Baffinland ; 9, Akpatok Island ; 10, Anti- 

 costi ; 11, New York — Canadian Ordovician province ; 12, New Jersey ; 

 13, Cincinnati region : 14, Tennessee region; 15, Minnesota; 16, Mis- 

 souri ; 17, Arkansas , 18, New Mexico. Stereographic projection, after 

 Penfield. 



* Platystrophia occurs in the Wisby (Unter-Mergel) formation of Gotland 

 (Schmidt), associated with Bilobites biloba, Rhipidomella hybrida, Stro- 

 phomena depressa, Rhyachotreta cuneata, etc., and in the lower part of the 

 Middle Gotland formation. These beds are equivalent, according to Lind- 

 strom, to the Wenlock of Britain (Niagara of this country). See Schmidt, 

 Archiv. Liv.-, Ehst.- und Kurlands, ser. i, ii, 2, 1854, pp. 426, 453, 454 ; and 

 Lindstrom, Of. Sven. Vet. Akad., xvii, 1860, p. 381 ; Neues Jahrbuch f. Min. 

 Geol. u. Pal., 1888, i, pp. 147-164. 



f An occasional individual from the Clinton of Rochester, N. Y., presents 

 somewhat the appearance of P. dentata, but with a larger number of plica- 

 tions and a shallower sinus. They are probably the young of P. biforata. 



