2cS SaMITH. [Vol. XVI. 



Schloenbachia oregonensis in its development repeats the 

 history of Anarcestes, Parodoceras, and Prionoceras in the first 

 five septa and one-quarter of a coil from the nautiloid proto- 

 conch ; then for about one whorl it is a Glyphioceras ; for about 

 one and one-quarter whorls it is a Gastrioceras ; then for a little 

 more than one-quarter of a revolution it is a Paralogoceras, and at 

 two and five-eighths coils ends its goniatite history, takes on a 

 keel, and becomes an ammonite, but one like the simpler ammo- 

 nites of the Permian and Lower Trias. The ananeanic stage 

 lasts up to three and three-eighths whorls, that is, about three- 

 quarters of a revolution ; the metaneanic stage lasts up to the 

 end of the fourth whorl, and the paraneanic to near the end of 

 the fifth whorl. With the beginning of the sixth whorl, at 

 diameter of about 12 mm., the shell begins to take on its own 

 proper characters, and is then in the ephebic stage, although 

 adults grow to at least 30 mm. in diameter, and probably larger. 



The larval stages may be compared with considerable cer- 

 tainty to ancestral Paleozoic genera, but the Mesozoic genera 

 to which the adolescent stages might be compared are probably 

 mostly unknown as yet, although they will be found among 

 trachyostracan descendants of the Glyphioceratidae, and not 

 among the Prolecanitidae. 



No more striking demonstration of the law of acceleration of 

 development, or tachygenesis, is possible than where a shell in 

 its larval history hastens through, in two and five-eighths whorls, 

 and in growth up to 2.70 mm., generic changes from Anarcestes, 

 Parodoceras, Prionoceras, Glyphioceras, Gastrioceras, and Para- 

 legoceras, an amount of development that its ancestors required 

 the time from the Lower Devonian to the end of the Car- 

 boniferous to accomplish. In the succeeding adolescent stages 

 the changes are not nearly so rapid. 



Another fact brought out by the investigation of many 

 specimens is that individual variation increases greatly with 

 the advance of the stage. Thus all protoconchs and most 

 chambered stages are alike up to the end of the larval period. 

 After that the uniformity ends, for in the adolescent period the 

 ribs begin at various sizes, as does also the digitation of the 

 septa. And in the acquirement of adult characters still greater 



