Cu mings — Morph ogen esis of Pla tystrqph ia . 1 3 



Of these gerontic modifications, the thickening of the shell 

 and obsolescence of plications are the most important. The 

 former character is well shown in fig. 14, and the latter in fig. 

 12. The thickening of the valves is responsible for the 

 increasing incurvature of the beaks. In extreme cases (fig. 5 A) 

 this causes the pedicle to encroach on the ventral beak to such 

 an extent as to completely abandon its normal foramen, which 

 is invaded by the dorsal beak. 



Fig. 5 represents the largest specimen of P. lynx seen. C 

 shows it in its natural position during life with an indication of 

 the musculature. This individual is decidedly senile, complete 

 ephebic growth having terminated at the varix No. 7J though 

 the amount of thickening of the shell is not as great as in 

 many smaller individuals (cf. fig. 5B and fig. 14Z?). 



PI (dystrophia aciitilirata presents even more extreme 

 gerontic modifications than P. lynx. The greatest gibbosity 

 seen in any group occurs here, and the amount of change in 

 the shell index is truly remarkable. It will be more con- 

 venient, however, to discuss the details of these changes due 

 to senescence under the heads of the several species and 

 varities of Platystrophia. 



Platy atrophia lynx* 



The species commonly known in this country as Platystro- 

 phia lynx is beyond any question the most abundant and wide- 

 spread Ordovician member of the genus, and also bears the 

 greatest resemblance to the species commonly known as Platy- 

 strophia hiforata (Schlotheim). Hall f does not attempt to 

 distinguish between them. Davidson % says in regard to the Brit- 

 ish specimens, " We certainly have the type hiforata and the 

 variety lynx, but these two seem so intimately connected that 

 I have combined them under Schlotheinrs single designation 

 \biforatd\P According to von Buch,§ lynx has four plica- 

 tions in the sinus and on the fold] and nine on each side. 

 Schlotheim' s hiforatus, he says, is " very nearly related " to 

 lynx. The former has five plications in the sinus and nine on 

 each side and is broader than lynx. According to de Yerneuil^f 

 " The S. [Spirifer] hiforatus proper seems to be rare ; we 

 have, as had Eichwald, a single specimen very poorly pre- 

 served. His came from Reval and ours from the neighbor- 



* Eichwald, Skizze von Podolien, 1830, p. 202. 



f Pal. N. Y., i, 1847. p. 132. 



X Silurian Brachiopoda, 1871, p. 271. 



g Essai d'une classification et d'une description des Delthvris, etc., Mem. 

 Soc. Geol. France, iv, 1840, pp. 190, 191. 



I He certainly was mistaken in either the number on the fold or in the 

 sinus. There is always one more on the fold. 



1 Geol. de la Eussie, 1845, pp. 136, 137. 



