AUCELLA. 203 



Ijut are decidedly different in extreme examples. It is usually the case also 

 that one variet)* will be found to prevail in certain layers of rock, some- 

 times almost exclusiveh*, and the other variety in other layers. 



Adult examples of one of these varieties are large, robust, and often 

 inflated. These approach the typical forms of A. concentrica more nearly 

 than the' others. Those of the other variety are smaller, more slender, and 

 have a more delicate appearance. They seem to cori-cspond more nearly 

 with the ty|)e of ^1. mosqueitsls. Still, after examining numerous examples 

 from Alaska, British America, Washing-ton Territory, and California, 

 besides some Russian examples oi A. concentrica and A. mosqnensis, believed 

 to be authentic, in the collections of the Smithsonian Institution, Dr. White 

 is of the opinion that all of them represent only one species. Indeed, he is 

 disposed to regard as at most only varieties of one s})ecies all the forms 

 which have fvom various authors received the names Aucella concentrim, 

 A. mosquensis, A. pallasii, A. crasskoUis, A. riochii, and A. Errinfjhmii. 

 However, it will be convenient, when discussing the AnceUa-hc^v'mg strata 

 of California, to retain the names A. concentrica and A. mosqmnsis to indi- 

 cate the more robust and the more elongate forms, respectively, as they 

 occur in that State. 



Before dismissing this reference to Amelia, it is well to note how wide 

 \i the geographical distribution of the variable form which has been known 

 under the various names which have just been mentioned. This shell was 

 first known in various parts of Russia and subsequently upon the eastern 

 coast of the Caspian Sea,^ in northern Siberia,' on the island of Spitzbergen,^ 

 on Kuhn Island (off the east coast of Greenland),^ and in Alaska, Britis!i 

 Columbia, Washington Territory, and southward to central California, as 

 mentioned on previous pages. Although it is so variable in certain of its 

 features, so constant is it in its general characteristics and so distinct from 

 related forms that paleontologists are now generally agreed as to its identit\- 

 in all the widely separated localities which have just been indicated. 



'See Eicliwald's Geognost.-palaeont. Bemerkuiigen iibor ^lio Halbiusel Mangiscblak iiml die 

 aleutischey Inselii, 1871, p. 53. 



= See Middea<lorft"s Reise iu deu iiussersteu Nordeu und Osteu Siberious, vol. 1, part l,p. 255. 



'See Liudstriini: Oiu Trias- ocli Jurafcirsteuingar fiaii Spetsbergen, Kongl. svensk. Vet.-Akad. 

 Haudl., vol.'t), No. 6, 18G7, p. 14. 



<See F. Toula, Die zweite deutsche Nordpolarfalirt, vol. 2, 167J, pp. V.)~-'M:>; al.so Quart. Jour. 

 Giol. Soc. Londiin, vol. :!4, 1876, p. 560. 



