Cum ings— Morph ogen esis of Platy strop hia. 45 



for Mr. Meek, Pander's 0. dentata not being sent." It seems 

 that Meek received the labeled specimens from James. The 

 latter further says : " I now propose the above name \_crasscc\ 

 for the shell described by Meek [as dentata Pander]." Meek 

 (loc. cit.) expresses grave doubts as to the identity of the form 

 with Pander's species. He says : "I doubt very much, how- 

 ever, whether it agrees with that variety [dentata], which is 

 described by McCoy, from British specimens, as having con- 

 stantly two plications in the sinus." Nevertheless, Miller* 

 subsequently referred the form to dentata without further 

 remark. It is known now that Meek's dentata is the same as 

 crassa James ; and it has been pointed out that both are the 

 same as costata Pander. The name crassa therefore lapses 

 into the synonomy of costata. 



Platy atrophia dentata Pander is a perfectly distinct type 

 found in the Ordovician of Russia and the Niagara of both 

 Europe and America. To this type belong the Anticosti and 

 Gotland forms, as well as the Niagara form of Kentucky and 

 Indiana. 



In regard to the variation of P. dentata, there is little to be 

 said. Shaler's measurements of a series of twenty shells from 

 Anticosti (which are now before me) show but little variation, 

 and the same is true of the Gotland specimens, abont thirty of 

 which I have measured. There is also scarcely any variation 

 in number of plications. One specimen from Gotland has 

 three instead of two in the sinus, though only two start at the 

 beak. The Kentucky specimens figured by Nettleroth also 

 have at the front margin more than the usual number of plica- 

 tions in the fold and sinus, f but otherwise closely resemble the 

 Gotland form. 



As to the derivation of P. dentata, the same argument that 

 derives the closely related P. biforata from a primitive unipli- 

 cate stock holds in the present case. P. dentata was derived 

 from such a primitive stock, probably at about the same time with 

 P. biforata, and by the same process of acceleration of the 

 point of bifurcation of the primary plication of the sinus. The 

 transitional stages are to be sought in Russian deposits, since 

 dentata is absent from the American Ordovician. 



In Niagara time the species spread westward into Gotland, 

 and from there into England and Ireland and the Gulf of St. 



* Cin. Quar. Jour. Sci., vol. ii, 1875, p. 27. 



f On the Anticosti form see Shaler, Bull. Mus. Comp. Zool., iv, 1865, p. 67, 

 and Brachiopoda of the Ohio Valley, p. 44. This is the form to which he 

 gave the name regularis. On the Kentucky form see Nettleroth, Kentucky 

 Fossil Shells, 1889, p. 35, pi. 29, figs. 18, 19. 



