418 JAMES PERRIN SMITH 



by the writer, must have descended not only from the same 

 perisphinctoid family, but also from the same species of Hop- 

 lite s ; and thus, if the parallel were at all exact, they should 

 be alike in the late adolescent stages when they begin to show 

 their generic characters. This, however, is not the case, for they 

 are quite different throughout the cosmoceran stage, and back 

 almost to the end of the larval period, where the transition from 

 goniatite to ammonite took place. If this were interpreted 

 without taking account of unequal acceleration, it would seem 

 that the differentiation of the two species took place back in the 

 Trias, and that different asgoceran forms were the remote ances- 

 tors of the two species, which we know could not have been the 

 case. 



The writer has recently worked out the ontogeny of two 

 very nearly related species of Schlcenbachia, one of which, in its 

 larval period, reproduces very exactly a Paralegoceras stage, 

 while the other does not ; the latter species has, however, all the 

 paralegoceran characters, but associated with others that this 

 genus never had, but which belonged to later descendants of 

 this genus. There can be here no question of the veracity of 

 nature in keeping the record, the difficulty lies in deciphering it. 

 So it is not to be expected that any one species would give in 

 plain terms the complete phylogeny of a genus, for stages that are 

 plainly differentiated in one will be obscured in another, and only 

 by studying the ontogeny of a number of species of one genus can 

 the morphologist hope to get a complete history. It is still less 

 to be expected that two separate genera, even when closely 

 related, should tell their story in exactly the same terms, for 

 stages that are emphasized in the ontogeny of the one are 

 obscured and possibly even omitted in the other. And in this 

 case the unequal acceleration goes much farther than with 

 closely connected species. In a comparative study of Lytoceras 

 and Phylloceras the writer 1 has recently shown how rapid this 

 divergence is, and has drawn the conclusion that unequal 



1 The Development of Lytoceras and Phylloceras. Proc. Calif. Acad. Sci., Third 

 Ser. Geol., Vol. I, No. 4, 1898. 



