14 ACCELERATION OF DEVELOPMENT IN FOSSIL CEPHALOPODA 



of Metasibirites, Homerites and Leconteia, illustrates clearly arrest of de- 

 velopment, with accompanying retardation of characters, and partial re- 

 version to ancestral types. 



Metasibirites is a dwarf, degenerate genus, confined to the Upper 

 Triassic, in India, California, and the Alps. The ontogeny of only the 

 American species has been published, but the statements made here are 

 based on the development of several American species, of which only two 

 have been described, Metasibirites (Tardeceras) parvus H. and S., and 

 M. (Tropiceltites) Frechi H. and S. (PI. VII, figs. 1-10). In early youth 

 Metasibirites Frechi is a typical Gastrioceras, with broad low trapezoidal 

 whorls, strong umbilical knots, frequent constrictions, and simple gon- 

 iatitic septa. Towards maturity the whorls become higher, the sculpture 

 begins to run up the sides, and ribs begin to develop from the knots, 

 which themselves become weaker and often obsolete. These ribs run up 

 to and finally across the venter. But before this is complete a weak keel 

 appears, bounded in some cases by weak furrows. The keel speedily be- 

 comes obsolete, often disappearing entirely at maturity. When nearly 

 mature the shell in nearly all its characters is a minature of Acrochor- 

 diceras of the Middle Triassic, but the septa remain goniatitie, or at least 

 only very weakly serrated. 



This is not a persistence of Sibirites from the Lower Triassic, but an 

 arrest of progress so that some of the characters fail to get beyond the 

 complexity that they had in that genus. In other characters the genus 

 has gone beyond Sibirites, in some respects even fallen short of it. The 

 genetic series of adult forms is as follows : Pericylus of the Subcarbonif- 

 erous developed into Gastrioceras, which in turn changed over into Sib- 

 irites of the Lower Triassic; this by gradually increasing strength of 

 sculpture and increasing complexity of septa developed into Acrochor- 

 diceras. There the stock became partly degenerate and development was 

 arrested : The forms affected failed to grow up to the size and complex- 

 ity of the immediate ancestor, Acrochordiceras, but stopped nearly in the 

 Sibirites stage of development, and in some characters even reverted to 

 the more remote ancestor, Pericyclus. 



The tendency to form a keel was strong in nearly all the groups of 

 the Tropitoidea, and crops out weakly here in the temporary develop- 

 ment of the vestigial keel. No member of Sibirites or Acrochordiceras 

 ever possessed a keel, so its development in Metasibirites can hardly be 

 charged to palingenesis of this character by heredity from some long dead 

 Lower or Middle Triassic form. It is rather a manifestation of a latent 



