EEMARKS ON THE GENUS AUCELLA. 229 



limits which may be reasonably assumeil as those of a single species. Professor von 

 Zittel recognized the close relationship of the New Zealand form with A. concentrica, 

 I found the Brazilian form to differ from the latter in hardly a greater degree, and 

 Btoliczka's Indian species is evidently closely like certain varieties of A. concentrica. 

 It may be, therefore, that we ought to regard this relationship as extending to the 

 Aucellas of the southern hemisphere and possibly also to the Indian form, although 

 t!ie latter comes from strata which we seem bound to regard as of considerably later 

 age than the others. 



Admitting this close genetic relationship of all the known forms of Aucella, it is 

 necessary to further coniilude that they have been dispersed from some geographical 

 center. The only published reference to such dispersion that has come to my notice is 

 a brief suggestion l)y Mr. A. Pavlow that in Kus.sia they were derived from the north,' 

 but this does not fully meet the broader question of circumpolar and still more extensive 

 distribution. 



Having been dispersed from a single geographical center, the strata which bear 

 the remains of the original colony are necessarily older than those which bear the rc- 

 maius of the colonies which were last established before the extinctiou of the genus to 

 the extent of the time which was occupied by the dispersion and colonization. If, 

 therefore, the dispersion was primarily from the north, the -northern Aucella-hearing 

 strata are necessarily older than the more southerly ones, and, if subsequent dispersion 

 was from the eastern to the western hemisphere, the eastern strata referred to are neces- 

 sarily older than the western. I think our present knowledgeof this subject is too meager 

 to warrant any definite statement as to the directions in which dispersion has occurred, 

 but the geographical distribution of this assumed single species is so extremely wide 

 and its climatic range has been so very great that the time required for its dispersion 

 may easily have extended from the closing epoch of the Jurassic into the Neocomian 

 epoch of the Cretaceous, and there are apparently good reasons for believing that such 

 was really the case. 



The genus Aucella has been regarded by the majority of authors who have written 

 upon it as diagnostic of the Jurassic age of the strata which bear it; but certain 

 authors whose opinions are worthy of consideration are equally confident that all such 

 strata should be regarded as of Neocomian age. Professor von Zittel refers his A. 

 plicata from New Zealand to the "Jura or Lower Cretaceous." The form described 

 by mo (op. cit.) from the province of Scrgipe, Brazil, under the name of .4. hraziUenis, 

 is from strata that I have referred to the Neocomian and there seems to be no possible 

 reason to (luestion the Cretaceous age of the Indian species described by Stoliczka. 

 It is Professor Eichwald more especially who has contended for the Neocomian age of 

 the Aucella hifAviug strata of Europe and northern Asia, and he also makes the same 

 claim for those of Alaska. Mr. Whiteaves (op. cit.) is equally confident of the Cre- 

 taceous age of the Auci //^bearing strata of British Columbia. In California, although 

 a part of the strata which \i&a.v Aucella have been referred to the Jurassic, those which 

 bear these shells most abundantly have been referred by all the geologists who have 

 studied them to the Shasta group of the Cretaceous series, and there seems to be no 

 good reason to doubt the correctness of that reference. 



I Bull. Soc. g6ologique France, 3(1 series, vol. 12, 1S84, pp. 6S6-69G. 



