Oumings — Morphogenesis of Platystrophia. 41 



pauciplicate lynx. For the European form I can not speak. 

 It may even prove to be distinct from ours, though it certainly 

 seems to be identical. The wide distribution of costata would 

 seem to entitle it to rank as a species, yet the evidence at pres- 

 ent available is rather in favor of regarding it as a variety of 

 P. ly?ix, the differentiation of which began quite early in the 

 history of the genus. 



Platystrophia hiforata. — The reasons for considering this 

 species as distinct from American Ordovician forms of Platy- 

 strophia have already been stated, and the main features of 

 the type described. Little can be added in regard to the 

 foreign forms. The Russian hiforata occurs in beds as low 

 as the Trenton, and in this country, a shell of quite similar 

 aspect, and of unquestioned biplicate type, is found in the 

 Trenton of the Lake Cham plain region* associated with the 

 normal lynx type.f This association is of great interest since 

 it shows that the differentiation of these two species was going 

 on in the early Ordovician, and also that there was free inter- 

 communication between the American and European Province 

 at that time.J The total absence of hiforata from American 

 deposits throughout the remainder of the Ordovician might 

 also indicate a closure of this avenue of communication early 

 in that period.§ 



In the Clinton of this country we again meet with a biplicate 

 Platystrophia strikingly similar to the Russian P. hiforata. 

 The writer has examined about 50 individuals from Rochester 

 and Gasport, N. Y., Dayton, Ohio, Irvine, Kentucky, and 

 Richmond, Indiana. These present considerable variation, 

 mainly in the number of plications, which may be all the way 

 from 16 or 18 to 46 on the valve, a greater range than in any 

 other tyjje (see fig. 21). 



* Mr. P. E. Raymond sent me specimens from the Crown Point section. 

 The species occurs there in the upper part of the section. All the specimens 

 sent may be of the biplicate type, although several have three plications in 

 the sinus. The exfoliation at the beaks renders it impossible to make sure 

 to which type the latter belong. Billings figures a Canadian Platystrophia 

 with rounded cardinal angles and four plications in the sinus, two of which 

 appear to arise at the apex. This specimen is very similar to several of the 

 Crown Point forms, and may very likely belong to the same type. See 

 Eaymond, Bull. Am. Pal., No. 14, 1902, pp. 27, 28; and Billings, Geol. 

 Canada, 1863. p. 167, fig. 149 a. 



f Some specimens collected at Ft. Cassin, Vt., by Mr. H. M. Seeley, are 

 clearly referable to P. lynx. 



X See Freeh, Lethsea geognostica, Theill. Lethaea Paleozoica, 1897, 2, p. 100. 

 Ruedemann, Bull. N. Y. St. Mus., No 42, vol. 8, 1891, pp. 561-564; Ibid., 

 No. 49, 1901. pp. 104-107 and footnote p. 107. Ulrich and Schuchert, Bull. 

 N. Y. St. Mus., No. 52, 1902, pp. 633-663. 



§Cf. Dana, Man. Geol. 4th ed., 1894, p. 536. — Ruedemann, Am. Geol., June, 

 1897, pp. 367-391. 



