228 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



In the adolescent period Placenticeras goes through at 

 first a stage corresponding to Cymhites, or at least some 

 Cymhites-YC^^ form, of the Upper Trias; then to some 

 £egoceran genus of Upper Triassic or Lower Jurassic age; 

 then to some one of the earlier perisphinctoid genera; then 

 to Cosmoceras of the Jura, and lastly to Hoflites of the 

 Cretaceous. It is thus demonstrated by ontogenic study 

 that Placenticeras developed out of Hoflites, and thus be- 

 longs with that group near the Stephanoceratidee, and not 

 under the Amaltheidae, with which it is classed in nearly all 

 text-books. This relationship is shown also by the number 

 of ammonite species intermediate in character between 

 Ho-plites and Placenticeras, although they are conventionally 

 grouped under one or the other genus. 



It is a mistake to class Sphenodiscus as a subgenus under 

 Placenticeras, for it is neither ancestor nor descendant of 

 that genus, but a parallel, independent development from 

 the common stock Hoplites. The same retardation that 

 caused the peculiar arrangement of the lobes in Placentice- 

 ras has gone even further towards simplifying the septa of 

 Sphenodisciis, although this can not correctly, in either case, 

 be ascribed to reversion, since while both fail to reach, in 

 some respects, the full development of their ancestors, they 

 do not return to the characters of any of their predecessors. 



A parallel study of the ontogeny of two closely related 

 species shows that the results must be interpreted with cau- 

 tion. Placenticeras pacijicuni and P. californicum certainly 

 came from the same group of Hoplites, and probably from 

 the same species, but in the late adolescent stage they are 

 unlike, just as easily distinguishable as at maturity, owing 

 to pushing back of specific characters into the adolescent 

 period. Even in the Cosmoceras stage the two are quite 

 distinct; the difference becomes less in the perisphinctoid 

 stage, and undoubtedly the larval stages would be precisely 

 alike in both. But this does not mean that the line of de- 

 scent was the same only through the larval stages, and that 

 the two species branched out from different perisphinctoid 

 forms, for in all probabihty the perisphinctoid, cosmoceran, 



