2< » ( '"in ings — Morphogenesis of Platystrophia. 



sinus to bifurcate manifested itself as an adult character in the 

 ancestors of Platystrophia biforata, and that, subsequently, 

 acceleration crowded back the point of bifurcation into early 

 neanic stages. 



The most primitive Platystrophia should, as stated in another 

 place, be uniplicate, and it was from.such a uniplicate stock 

 that both P. lynx and P. biforata, were derived. The fact 

 that in lynx there is no modification of the fold and sinus 

 until an advanced neanic stage, while in biforata such modi- 

 fication takes place, as explained, at an early neanic stage, 

 indicates that lynx is nearest to this primitive Platystrophia. 

 P. lynx certainly stands as the radical of all our Ordovician 

 for) its above the basal Trenton. 



Variation of Platystrophia lynx. — This subject has already 

 been investigated quantitatively and the results published.* 

 As that investigation applied to specimens from the Middle 

 Cincinnati group only, there is considerable to add in regard to 

 Trenton forms and especially in regard to some highly acceler- 

 ated forms from immediately beneath the Clinton limestone at 

 Richmond and other localities in Indiana. The Yevay material, 

 quantitatively studied, included the varieties laticosta and 

 costata, which were shown to pass by insensible gradations 

 into the normal lynx type.f 



The lynx group of the Trenton varies extensively in contour 

 and number of plications, as has often been pointed out.:}; 

 Fig. 8 shows the contour of a series of sixteen specimens, 

 taken at random, from Trenton Falls' material. The index 

 here varies from 1*2 to 1*82, the average being 1*42 and the 

 largest class 1*4. The angle between the hinge line and 

 lateral line of the shell (cardinal angle) varies from 80° to 

 110° ; the number of plications on the ventral valve from 

 17 to 24, and the number in the sinus from 2 to 6. No. 32, 

 fig. 8, is a composite of Nos. 1-16 inclusive and shows the nor- 

 mal Trenton Falls' type of shell. This shell has a cardinal angle 

 of 90°, with very slight auriculation.§ 



* Cumings and Mauck, A Quantitative Study of Variation in the Fossil 

 Brachiopod Platystrophia lynx. This Journal, July, 1902, pp. 9-16. 



f Mr. F. W. Sardeson has expressed the opinion that all the so-called 

 varieties of Platystrophia, in the Cincinnati group are distinct. 



A careful study and measurement of over 2,000 individuals and inspection 

 of some 8,000 others, has failed to establish a specific distinctness. See 

 Sardeson, Am. Geol., vol. xix, 1897, p. 109. Compare also Williams, Geol. 

 Biology, pp. 315. 321. 



\ See especially Hall, Pal. N. Y., vol. i, 1847, pp. 133, 134. Winchell 

 and Schuchert Geol. Minn., iii. 1893, pp. 456, 457. 



£ When there is any auriculation of the cardinal extremity the measure- 

 ment is made between the hinge line and a line tangent to the cardinal ex- 

 tremity and the lateral margin, as shown in fig. 8 bis. 



