3-± Cum mgs — Morphogenesis of Platystrophia. 



and a normal P. laticosta there is, as above stated, every 

 possible gradation,^ there is between the latter and P. lynx* 

 P. acutilirata presents considerable variation, chiefly, how- 

 ever, in becoming first progressively more and more acuminate, 

 and second in becoming, in the upper part of its range, retro - 

 gressively less and less acuminate. The shell index varies in 

 progressive series from less than 1*6 to more than 2*7 and in 

 retrogressive series from the latter figure to 1*4. The number 

 of plications varies from 18 or 19 to 37 or 38, of which three 

 are almost invariably in the sinus (four on the fold).f The 

 cardinal angle varies in progressive series from 80° to 40° and 

 in retrogressive series from 40° to 90°, which is very nearly 

 the normal angle for P. lynx. These changes may frequently 

 be noted in a single individual. Fig. 18, 6, shows a retrogres- 

 sive individual from the upper Richmond beds of Richmond, 

 Indiana.:}; This specimen is inequilateral through inequality 

 of growth and not as a result of deformation after fossiliza- 

 tion. Such lack of symmetry is frequently met with among 

 these retrogressive individuals. The early ephebic stage (I) is 

 almost identical with such a form as fig. 17, b. The normal 

 acutilirata stage is shown at 7Z(fig. 18, d, illustrates the same 

 thing). The cardinal angle here changes from 76° in stage 7, 

 to 50° in stage 77, back to 77° in the final stage (using the right- 

 hand angle in each case). Fig. 18, f to k with the degrees 

 marked to the right, indicate the changes for other individ- 

 uals. Fig. 18, #, represents a specimen from the lower part of 

 the range of acutilirata.^ Here all the stages are progressive, 



* Prof. D. W. Dennis has called attention to the change from narrow to 

 greatly extended forms of P. acutilirata in traversing the Richmond section 

 from the foot to the head of the gorge of the Whitewater river. He did 

 not, however, recognize the relation of acutilirata to laticosta. Dr. Dennis 

 also calls attention to the variation of Hebertella in the same section. I 

 have already mentioned the presence of H. sinuata in the upper part of the 

 Richmond section. This species is also found at the base of the Richmond 

 beds and is there modified into the variety occidentalis. The latter is not 

 however modified back into a sinuata but this form comes in suddenly at the 

 top of the section. See Proc. Ind. Acad. ScL, 1898, pp. 288, 289. 



f Meek says the breadth is " sometimes twice, or even, in extreme cases. 

 three times the length of the valves;" and that there are "on each side of 

 the mesial fold and sinus from 11 to 18 [plications], making the entire num- 

 ber about 26 to 40 on each valve." I see no reason to doubt his extreme 

 figures. See Meek, Pal. Ohio, vol. i, 1873, p. 119. 



\ The retrogressive forms of acutilirata figured in this paper are nearly 

 all from the falls of the west fork of the Whitewater river, one mile N.W. 

 of Richmond. These beds come stratigraphically immediately below those 

 of Elk Horn creek, mentioned above as containing a peculiar type of P. lynx. 

 The latter beds are not exposed on the west fork. Fig. 11, c, is from equiv- 

 alent strata just south of Weisburg Station. Ind. 



§ This specimen is from Tanners creek, Dearborn Co. , Indiana, one mile 

 S.E. of Weisburg. It is associated with Prasopora hospitalis, Batostoma 

 varians, Leptcena rhomboidalis, Strophomena planumbona, Rafinesquina 

 alternata, a very convex form, and a very large form, Hebertella sinuata, 

 Streptelasma corniculum, etc., etc. 



