Cumings and Mauck — Variation in the Fossil, etc. 



Art. III. — A Quantitative Study of Valuation in the Fossil 

 Brachiopod Platystrophia lynxf" by Edgar R. Cumings 

 and Abram Y. Mauck. (With Plates 11 and III.) 



During the year 1900-01 the authors made extensive collec- 

 tions of the brachiopod Platystrophia lynx from the Upper 

 Ordovician rocks at Yevay, Indiana (Switzerland Co.). 



Inasmuch as this species is extremely variable and at the 

 same time exceptionally abundant and well preserved, a quan- 

 titative study of the variations presented at once suggested 

 itself. Such an investigation is the more warranted because of 

 the different opinions current as to the taxonomic importance 

 of several of the forms under which Platystrophia presents 

 itself.f 



The specimens used in this investigation were collected from 

 a zone which at Yevay extends from 240 ft. to 360 ft. above 

 the level of the Ohio River. Part of the material was so col- 

 lected that the precise layer is known from which a given 

 specimen came ; the object being to determine as accurately as 

 possible the factor of range.:}: The majority of the specimens 

 used are from the upper 50 or 60 feet, and the upper 20 feet 

 contains by far the most as well as the best of the material. 

 Examples could not be obtained in sufficient abundance from 

 the base of the Platystrophia zone to make a quantitative 

 treatment possible ; but we were able to ascertain that in the 

 lower part of its range at Yevay, and in general throughout 

 the Ohio Yalley, Platystrophia presents less variation than 

 at higher horizons ; and that in the lower beds only the small 

 pauci-plicate form is present.§ 



Beside the material collected layer by layer, a large collec- 

 tion was made from the upper beds wherever exposures could 

 be found in the vicinity of Yevay. 



The data taken for study are : Patio of width to length of 

 shell (equals shell index) ; ratio of depth to breadth of sinus 



* Presented before Section E at the Denver meeting of the American Asso- 

 ciation for the Advancement of Science, August, 1901. 



I For svnomony and bibliography of Platystrophia see Davidson, Silurian 

 Brachiopoda, ISh, p. 268; Halland Clarke, Pal. N. Y., vol. viii, pt. 1, 

 pp. 200, 201 ; Schuchert, Am. Foss. Brachiopoda, Bull. U. S. G. S., No. 87, 

 pp. 808-310. 



:{:The Yevay section is published in the Am. Geol., vol. xxviii, Dec, 1901, 

 pp. 361-381. By reference to page 373 of that paper it will be seen that the 

 lowest specimens of Platystrophia (associated with Dekayia ulrichi) are small 

 and of a t\^e similar to var. laticosta or var. dentata Meek {crassa James). 

 The form lynx does not come in till within 80 or 40 feet of the top of the sec- 

 tion, 



§ Specimens have been examined from these lower zones at Cincinnati, 

 Ohio, Lawrenceburg, Aurora, Eising Sun, Yevay and numerous points on 

 Laughery Creek in Indiana. 



