GEOL.-VOL. I.] SMITH— PLACENTICERAS. 1 87 



The two species of Place^iticeras, of which the ontogeny is 

 described in this paper, must have descended not only from 

 the same perisphinctoid family, but also from the same 

 species of Hoplites; 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 through- 

 out the cosmoceran stage, and back almost to the end of the 

 larval period, where the transition from goniatite to ammon- 

 ite 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 segoceran forms were the remote 

 ancestors of the two species, which we know could not have 

 been the case. 



The writer (1899) 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 Paral- 

 egoceras stage, while the other does not; the latter species 

 has, however, all the paralegoceran characters, but associ- 

 ated with others that this genus never had, but which be- 

 longed 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. 



Retardation. — Another factor that makes it difficult to 

 correlate ontogeny and phylogeny is retardation of develop- 

 ment. Cope first recognized the principle, but in his writ- 

 ings confused it with unequal acceleration, and since his rea- 

 soning was purely theoretical the idea has never gained much 

 foothold in biologic philosophy. Cope's statement (1887, 

 p. 142) of the theory is as follows: "The acceleration in 

 the assumption of a character, progressing more rapidly than 

 the same in another character, must soon produce, in a type 



