Cuming s — Morphogenesis of Platystrophia. 47 



From the Niagara of Osgood, Indiana, I have a single speci- 

 men (^Schnehert collection) which is identical with the neanic 

 stage of the Gotland form. It may be an immature indi- 

 vidual. 



The presence of the Gotland type of Platystrophia dentata 

 in the Xiagara of the Ohio valley is of interest in connection 

 with the strong resemblance of the Silurian faunas of the 

 Chicago area to the Gotland faunas, pointed out by Weller,* 

 and indicates a westward movement of the latter through the 

 Hudson Bay route during Silurian time. In this connection 

 it is also to be noted that the PI 'a ty strop hi a of the Wenlock of 

 England and Ireland, judging from , Davidson's ngures,f is 

 of the same type as that of Anticosti, that is, larger and less 

 angular than the Gotland form. The relations of these several 

 geological provinces will be readily understood by reference 

 to the map, fig. 24. The extremely small size and simplicity 

 of the Gotland and Osgood specimens is of special interest. 

 Some of the former have in the adult but four plications on 

 the lateral slope, and are therefore strictly comparable to the 

 neanic stage of the normal Ordovician types. Such a com- 

 plete return to the primitive type is rarely seen (see ~No. 16, 

 and xix, tig. 27). 



Platystrophia dentata is fully entitled to rank as a species, 

 although in the Ordovician deposits of Russia, according to de 

 Yerneuil, it is connected by intermediate forms with P. bifo- 

 rata. Its distinctness and wide distribution during the Silurian 

 are facts of greater import than the presence of such linking 

 forms in the Ordovician.;}; 



Abnormal and Pathologic Types. — Xo truly pathologic 

 shells of this genus have come to the writer's notice, with the 

 exception of a series of specimens submitted by Mr. Charles 

 Schuchert of the U. S. National Museum. These shells, two 

 of which are shown in fig. 25, are from the Richmond beds of 

 TVaynesville, O. They are all small, frequently distorted, as 

 in tig. 25. a : and show a pronounced tendency to obsolescence 

 of the plications at varying stages of growth. In fig. 25, a, this 

 tendency does not manifest itself till late in the ontogeny, after 

 the formation of a very conspicuous growth varix. In fig. 25, b, 

 the plications become obsolete a short distance from the apex 

 of the beak, and farther forward regain nearly their normal 

 strength. 



* Jour. Geol., vi. 1898. p. 697. Cf. Dana, Man. Geol.. 4th ed.. 1894, p. 

 536. De Lapparent. Traite Geol., 1900. pp. 809, 823 (Maps). 



+ Silurian Brachiopoda. 1869. pi. xsxviii, figs. 11, 20. 24. 



X It may be well when these early linking forms of P. biforata and P. den- 

 tata are more carefully studied to distinguish them by a varietal or sub- 

 specific name, as I have done in the case of P. laticosta. 



