JAMES PERRIN SMITH 9 



mercy of enemies. Thus, even after the egg-stage, characters will be 

 eliminated, or, at any rate, so obscured that they can not be recognized. 

 So the diagram should show a constant shortening or eliminating of stages 

 at the lower end of each ontogeny, corresponding to the egg development. 

 It should also show a constantly increasing length of ontogeny, probably 

 not in time, but in the number of stages gone through, and hence, by in- 

 ference, an ever increasing rapidity of development. From this idea 

 came Hyatt's name " tachygenesis. " Thus, for example, the develop- 

 ment becomes successively more complex in Bactrites, Anarcestes, Gonia- 

 tites, Gastrioceras, Columbites, Tropites, all steps in the same series, even 

 with the complete elimination of the earlier stages; while the actual 

 length of the larval stage was probably not greater in Tropites than in 

 Bactrites. Mesozoic genera, as a rule, show scarcely any reminiscences of 

 ancestors older than the Carboniferous, except in the case of fixed or left- 

 over types, such as Lecanites, which has persisted into the Middle Triassic 

 with characters little in advance of its Devonian ancestor. 



Paleozoic and early Mesozoic genera repeat, in their ontogeny, their 

 ancestral history with a fair degree of exactness, for they are not yet 

 greatly affected by unequal acceleration of development, and scarcely at 

 all by retardation or arrest of development. Their ontogeny is beauti- 

 fully simple and direct, and in them it is easy to find genetic series of 

 adult genera with which to compare the ontogenic series of stages in any 

 species. Such simple development and positive recapitulation is shown in 

 Goniatites of the Carboniferous, (PL I, figs. 1-9) ; Cordillerites, (PI. XII, 

 figs. 1-8), and JJssuria, (PI. XI, figs. 1-14), of the Lower Triassic. Dis- 

 tinct recapitulation with considerable acceleration is shown in the onto- 

 geny of Columbites of the Lower Triassic, (PI. IV, figs. 1-10) ; in the same 

 genetic series, Tropites, (PI. IV, figs. 11-21), of the Upper Triassic, shows 

 a recapitulation of nearly all the ancestral characters, but much obscured 

 by unequal acceleration, or "telescoping" of characters and stages of 

 development. 



In later Mesozoic genera the recapitulation of phylogeny in ontogeny 

 is not so distinct, since all the disturbing factors have combined to 

 obscure the record. All have still a goniatite stage at the beginning of 

 their larval development, but in Cretaceous genera it is no longer pos- 

 sible to point out with certainty the particular ancestral goniatite genus. 

 The young of all that have been examined resemble the Carboniferous 

 family Glyphioceratidas, which may mean that all Cretaceous ammonoid 

 genera came from that stock, or else, more probably, that the round form 



