CONCLUSIONS BASED ON STUDY OF AUCELLA. 407 



There seems to nie, therefore, to be good grounds for the following 

 conclusions : 



1. That, laying aside the vexed question of what constitutes a species, 

 there are here three distinct and probably connected forms, beginning 

 with and perhaps evolved out of Aucella erringtoni var. arcuata, or some 

 common form represented most nearly by it, which migrated along a 

 changing sea bottom in which sediments were accumulating. 



2. That as the sediments accumulated the Aucella erringtoni var. arcuata 

 gradually shifted its location chorologically northward and chronolog- 

 ically upward into the Knoxville basin, changing as it migrated into 

 Aucella inochi of the slender form. 



3. That in the same basin this species remained, migrating chronolog- 

 ically with the accumulation of sediments and gradually changing into 

 the stouter, constricted form known under the same name, but stated by 

 Messrs Diller and Stanton to occur only in the upper part of the Knox- 

 ville slates. 



I do not state here the exact lines followed in this migration, but only 

 the result in the final arrival of the modified form in the Knoxville 

 basin. 



Stoliczka's* figure of a form oi Aucella discovered in the Tagling lime- 

 stone, lower Lias of India, certainly appears to support this opinion, and 

 the discovery shows that this genus originated earlier in the Jura than 

 has been supposed and must have traveled northward in Asia, as it did 

 in this country. It is to be noticed also that this Liassic shell has fine 

 radiating striae like our own Upper Jurassic species in California. 



It will be remarked here that I take a position different from that 

 generally adopted in Europe and oppose the inference that the northern 

 Aucella? migrated to the southward. 



Whether there is nonconformity between the Knoxville and Gold belt 

 slates is of no consequence to this argument; it is just as sound in case 

 of conformity] as of nonconformity. If there is conformity, probably 

 varieties oi Aucella will be found having not only identical striations, but 

 other characteristics identical with those oi Aucella erringtoni var. arcuata 

 in the lowest part of the Knoxville group or below il in rocks not yet 

 explored. 



CONCLUSIONS DERIVED FROM A-'iSOCIATIONS AND COMPARISONS OF FAUNAS. 



These conclusions are based upon Aucella?, but the collateral evidence 

 of associated forms in the different faunas gives additional strength to 

 the same opinions. 



* Aucella leguniinosa, Stoliczka: Mem. Geol. Surv^ey of India, 1866, pi. 8, f. 8. 



