242 SMITH. [Vol. XVI. 



ably constant in all ammonites, but even in nearly related spe- 

 cies of the same genus the first larval stages may be quite 

 different, although the later larval stages may be very similar. 

 It is a well-known fact that free larvae have a much better 

 chance of repeating their ancestral history in unabbreviated 

 form than embryos that go through their development in the 

 egg.i It is quite probable that different species of ammonites, 

 just as is the case with living molluscs, were hatched at differ- 

 ent stages of growth, and that the omitted stages may corre- 

 spond to a longer period spent in the Q.gg by one species than 

 that spent by a species that did not omit these stages. Thus 

 certain species of Schloenbachia reach the glyphioceran stage 

 immediately after the protoconch, while others go through sev- 

 eral generic stages between the protoconch and the glyphioceran 

 stage ; this happens in species in the same geologic horizon, so 

 it cannot be due to difference in removal from the parent 

 radicle. Such cases as these make it hard to interpret ontog- 

 eny, but they are not "falsifications of the record." HolzapfeP 

 has described the young stages of Anarcestes karpinskyi Holz- 

 apfel, showing that it goes through a typical mimoceran stage, 

 in which the nepionic shell does not touch the protoconch for 

 half a revolution. But in the ontogeny of a nearly related ^"^q- 

 ciQS, Anarcestes plebeifortnis Hall, as described by J. M, Clarke,^ 

 this mimoceran stage is omitted, for the shell is close coiled 

 from the very beginning. We might say, however, that the 

 mimoceran character of the open coil is pushed back by unequal 

 acceleration and lost, while other mimoceran characters are 

 retained, though so merged with those of Anarcestes that it 

 is impossible to recognize them. 



Each ammonite went through a larval history that is long 

 and varied in direct projwrtion to the length of time from its 

 period back to the Lower Devonian, when the first of the race 

 are known. Thus in the Naiitilinidae, the first group of ammo- 

 noids, the ontogeny is comparatively simple, there being few 



1 Balfour, Treatise on Comparative Embryology, vol. ii, p. 362. 



2 " Die Fauna mit Maeneceras terebratum Sandberger," Abhandl. k. Pretis- 

 sischen Geol. Landesanstalt, N.F., Heft 16, p. 77, PI. Ill, Figs. 15-20, 1895. 



3 "Notes on the Early Stages of Certain Goniatites," ibth Ann. Rep. State 

 Geologist of New York, pp. 165-168, 1898. 



