PRINCIPLES OF PALEONTOLOGIC CORRELATION 693 



to split it up into a number of species, because there are transi- 

 tions between all the varieties. Dr. A. Tornquist 1 has recently 

 shown that Ceratites nodosusdXso occurs in the upper Muschelkalk 

 of the southern Alps, and that there the varieties lack the tran- 

 sitions, and thus may be given names to mark this transforma- 

 tion. The immediate varieties never reached the Alpine province, 

 or else the modification took place on the way. The zone of 

 Ceratites nodosus is thus inter-provincial in extent. 



A somewhat similar case is known in the distribution of cer- 

 tain living species of the genus Purpura; in the English waters 

 Purpura lapillus is common and exceedingly variable, but no 

 constancy can be traced in these variations, the influence of tem- 

 perature, sea bottom, and food supply being so evident, and the 

 transitions so gradual that no subdivision of the species is 

 attempted. Some of these same varieties are found on the 

 western coast of America, but without the transitions, and so 

 they are called by a number of specific names, which, although 

 they are given to forms locally distinct, can certainly be only 

 synonyms of Purpura lapillus. At some not very distant time 

 these forms migrated westward from the Atlantic waters, and 

 either varied on the journey, or else the intermediate forms did 

 not succeed in reaching the Pacific region. 2 Here is certainly 

 an interregional migration where a species is still living in the 

 waters where it originated. 



The genus Clymenia, according to J. M. Clarke, 3 appears in 

 the Goniatites intumescens zone in New York ; in Europe Clyme?tia 

 is wholly unknown in the Intumescens fauna, but is the character- 

 istic form of the next higher division of the Devonian, where 

 the Intumescens fauna was already extinct. In North America 

 Pronorites cyclolobus and Conocardium aliforme appear in the Lower 

 Coal Measures, while they flourished in Europe in the zone of 



1 Zeitschrift d. Deutschen Geol. Gesell. Bd L. Heft 2, 1898, and Heft 4. 1898. 

 Neuere Beitrage zur Geol. und Palantol. der Umgebung von Recoaro and Schio (im 

 Vicentin). 



2 A. H. Cooke, Mollusks, 1895, pp. 90 and 363. 



3 Am. Jour. Sci., Ser. 3, Vol. XLIII, p. 57. 



