44 ( '>! ni ings — Morphogenesis of Platystrophia. 



our Clinton forms.* The derivation of P. biforata has been 

 discussed at some length in connection with P. lynx. It is 

 plain that we must look to European deposits for the fullest 

 light on this point. I am convinced that whatever new evi- 

 dence arises will be found to support the position taken here, 

 that P. 1/ynx is really the more primitive. The history of the 

 species after its reappearance in our Clinton faunas is not as 

 fully understood as could be wished, owning to the small 

 amount of careful stratigraphic work that has been done on 

 this series of rocks. Too much emphasis can not be laid on 

 the necessity of accurate, detailed, and comprehensive work 

 of this sort. Nevertheless the main features outlined above 

 are not likely to be modified by future investigations. 



To Platystrophia biforata is here assigned the rank of a 

 species occupying the same position of prominence in relation 

 to the European and Silurian faunas that P. lynx does in rela- 

 tion to the American Ordovician faunas. So far as is now 

 known, the type did not survive the Clinton. f 



Platystrophia dentata.% — So much confusion exists in this 

 country in regard to this type of Platystrophia that it will be 

 necessary to enter somewhat in detail into the question of 

 synonomy. 



In 1830 Pander described and figured Poramboniies den- 

 tata, P. costata, P. orevis, and P. recta. The affinities of the 

 last two are uncertain, but the first two represent very charac- 

 teristic and widely distributed types of Platystrophia. P. den- 

 tata, according to both the figures and description given by 

 Pander, has two plications in the sinus (three on the fold) and 

 about five on each side. The sinus is deep, the contour of the 

 shell rounded, and the profile plump, or even gibbous. This 

 form was refigured and described by deVerneuil,§ who says 

 that it passes by insensible gradations into biforata on the one 

 hand and chama (=costata) on the other. Davidson] placed 

 dentata in the synonomy of biforata. In 1873 MeekT de- 

 scribed under this name a form which he says is "referred in 

 Mr. James's list to 0. \_Orthis~\ dentata of Pander." In regard to 

 this, Mr. James*- says, the specimens "were wrongly put up 



* According to Schmidt, fissicostata occurs in Russia at Muddis, Koil, 

 Lyckholm, and Hohenholin, in the Lyckholm beds, which are equivalent in 

 age to the upper part of our Cincinnati group. See Schmidt, Archiv. fur die 

 Naturkunde Liv.-, Ehst. - und Kurlands, Iter ser, 2ter Band, lte lieferung, 

 1858, p. 213. 



f Dclthyris brachynota Hall (Geol. 4th Dist. N. Y., 1843, p. 71) is a P. 

 biforata of the Clinton type. 



X Pander, Beitrage zur Geognosie des russischen Reiches, 1830, p. 96, pi. 11, 

 figs. 4 a-e. 



?• Geol. de la Russie, 1845, p. 138. pi. iii, fig. 5 a-f. 



|| Silurian Brachiopoda, 1871, p. 268. 



•| Pal. Ohio, i, 1873, p. 117. 



** Cin. Quar. Jour. Sci., i, 1874, p. 21. 



