Geol.— Vol. I.] SMITH— LYTOCERAS AND PHVLLOCERAS. 133 



second corresponded to Ihergiceras, metanepionic ; on the 

 third whorl was seen the Paraprolecanites stage, paranepi- 

 onic; at end of the third whorl the Pronorites stage, begin- 

 ning of the neanic; on the fourth whorl the Sicanites stage, 

 end of neanic; on the fifth whorl Promedlicottia, an- 

 ephebic ; and lastly, at end of the fifth whorl, Medlicottta, 

 adult in characters, though not yet in size. 



In a recent paper the writer ^ has shown that Glyphtoceras 

 in its development goes through ammonoid stages from 

 Anarcestes and Tornoceras as a larva, through Prionoceras 

 in the adolescent stage, until at diameter of six millimetres 

 it becomes a full-fledged Glyphioceras. 



Law of Acceleration of Development. 



The study of ontogeny would have little interest and 

 meaning if the law of acceleration of development were 

 unknown. This principle was first used by Louis Agassiz 

 as an aid in the systematic study of biology, but it was 

 reserved for Alpheus Hyatt to formulate the law, and to 

 strengthen theory with practical examples based on the 

 study of cephalopods.^ An exact and comprehensive defi- 

 nition of the law of tachygenesis is the following: "All 

 modifications and variations in progressive series tend to 

 appear first in the adolescent or adult stages of growth, and 

 then to be inherited at earlier and earlier stages according 

 to the law of acceleration, until they either become embry- 

 onic or are crowded out of the organization, and replaced 

 in the development by characters of later origin." ^ And in 

 a later paper Professor Hyatt has given a more definite 

 statement: "The substages of development in ontogeny 

 are the bearers of distal ancestral characters in inverse pro- 

 portion, and of proximal ancestral characters in direct pro- 



1 Proc. Cal. Acad. Sci., 3d Ser., Geol., Vol. I, No. 3. "The Development of Glyphioceras 

 and the Phylogeny of the Glyphioceratidse." 



2 Mem. Boston Soc. Nat. Hist., Vol. I, 1866-7; Proc. Boston Soc. Nat. Hist., Vol. I, 1866. 

 " Parallelism of Individual and Order among the Tetrabranchiate Mollusks." 



3 A. Hyatt, "Genesis ot the Arietidse," Preface, p. IX. 



