Cumings — Morphogenesis of Platystrophia. 133 



these nepionic plications would account for shell plication in 

 some instances ; and probably striae are usually due to the 

 presence of setse and other organs in the mantle margins. 



Progressive and retrogressive evolution. — The greater part 

 of the history of Platystrophia is progressive. There may 

 indeed have been retrogression in minor details accompanying 

 general progressive evolution ; as in the obsolescence of the 

 secondary plications of the sinus of P. laticosta and P. 

 costata. There was a return in this particular to a primitive 

 condition. In the later history of the group genuinely retro- 

 gressive series are met with, notably P. acutilirata (upper 

 Richmond form) and Bilobites various. P. lynx of the upper 

 Lorraine, we have seen, presents markedly gerontic characters 

 but scarcely gives rise to a truly retrogressive series. General 

 retrogression, in the Platystrophia group, in every case heralds 

 extinction. Characters added at various times during the 

 history of the genus are lost in the inverse order of their 

 acquisition. For example, in P. acutilirata the latest added 

 character is the acuminate cardinal angles. This is the first 

 feature to disappear in retrogression. In extreme retrogression 

 the sinus decreases in depth and angularity, the auriculation of 

 the cardinal extremities disappears, and the shell assumes a 

 markedly lynx-like aspect. Bilobites various, as pointed out by 

 Beecher,* first loses the pronounced bilobation that charac- 

 terizes jB. verneuilianus. It never loses the vestigial median 

 plication of the ventral sinus. The older a character, the more 

 persistent it is. 



In all these cases of retrogression the process seems to be 

 due to the acceleration of gerontic modifications. This is 

 especially well shown in P. acutilirata, where we first meet 

 with an occasional gerontic individual which has lost its acumi- 

 nate cardinal angles and finally in higher zones find the whole 

 group thus affected, and the gerontic modifications appearing 

 in early ephebic stages. I have noticed a precisely parallel 

 case in Sjpirifer mucronatus of the Devonian. Some of the 

 later forms of this species are so modified by the acceleration 

 of gerontic characters that the cardinal angles approximate 90°, 

 the more acuminate stages being indicated by the growth lines 

 on the posterior portion of the shell. The inverse order of dis- 

 appearance of characters applies here, also, as it seems to in 

 practically all retrogressive types. f 



Yale University Museum, New Haven, Conn., Nov., 1902. 



*This Journal, (3), xlii, 1891, p. 56. 



fFor other examples of retrogression, see the various papers of Beecher 

 and Beecher and Clarke on the development of the Brachiopoda, and Beecher's 

 chapter on the Trilobita in Eastman's translation of Zittel's Grundzuge. See 

 also Hyatt's numerous papers on the Cephalopoda, and Jack son's memoir on 

 the Phylogeny of the Pelecypoda, Mem. Bos. Soc. Nat. Hist., iv, 1890. 



