Geol.— Vol. I.] SMITH— PLACENTICERAS. 189 



every stage passed through by the higher genera has a fixed 

 representative in a lower genus. Moreover, the lower 

 genera are not merely equivalent to, or in exact parallelism 

 with, the early stages of the higher, but they express a per- 

 manent type of structure, so far as these genera are con- 

 cerned, and after reaching maturity do not show a tendency 

 to attain higher phases of development, but thicken the shell 

 and cardinal process, absorb the deltidial plates, and exhibit 

 all the evidences of senility." 



If, then, the morphologist tries to study the race history 

 in one of these species thus arrested in development, he can 

 not read the whole story, for the individual ontogeny will 

 not recapitulate the higher stages lost by retardation. 



Another remarkable case is that of the so-called " cera- 

 tites " of the Cretaceous. While there have been no 

 goniatites since the Paleozoic, and no ceratites since the 

 Trias, there are found among the ammonites of the Creta- 

 ceous some with septa of simple goniatitic character, and 

 others with septa like those of the genuine ceratites. Now 

 since the line of descent is broken and there is no possibil- 

 ity for a continuous line of these ancient primitive forms to 

 have bridged over the great gap from the Trias to the Upper 

 Cretaceous, we must explain this either by reversion, or in 

 some other way. But it is not a simple case of reversion, for, 

 as has been pointed out by several writers (Douville, 1890, 

 pp. .275-292; Nickles, 1890), the septum of adolescent am- 

 monites of this group is not more complex, but really less 

 so, than that of adults, although they are derived from Juras- 

 sic genera with complex septa. Thus Douville, in the paper 

 cited above, derives the group Placenticeras-S^henodlscus 

 from Hoplites; the PulchellidcC, composed of Piilchellia, 

 JVeolobites, and Tissotia, he derives from Of-pelia of the 

 Jura. Since in each case the ancestral forms are more 

 complex than the descendants, the reduction in complexity 

 of generic evolution can be explained only by retardation, or 

 arrested development. F. Bernard (1895, p. 668) has, in 

 addition, pointed out the fact that the adult of Piilchellia is 

 like the adolescent stage of the ancestral Oppclia. Now, if 



