226 bulletin: museum of comparative zoology. 



Chiron limestone of Kinnekulle and Oeland, serves to indicate that 

 the upper part of the Orthoceras Hmestone in these locaHties is the 

 equivalent of the Xemagrapfus gracilis shale as well as the Geminus 

 shale. And the correlation of the Chasmops limestone with the grapto- 

 lite shales of the Dicranograptiis clingani and Pleurograptus linearis 

 zones is assured by the presence of several species common to the two 

 types of deposits. 



CORRELATION OF THE AMERICAN WITH THE 

 EUROPEAN FORMATIONS. 



In attempting a correlation over such a distance one is of course 

 obliged to depend very largely on fossils; and it is necessary to assume 

 an hypothesis which all the evidence seems to support, namely, that 

 cosmopolitan faunas reach their wide distribution within a very short 

 space of time. In the present case, it is necessary to depend very 

 largely upon the graptolites which seem to be more widespread than 

 any of the other organisms. Graptolites are, of course, almost absent 

 from Russia, so that it is necessary to correlate the Russian with the 

 Swedish sections by means of other fossils. The trilobites, being best 

 known, have been used most, but in certain cases species of brachio- 

 pods, cystids, or cephalopods have proved of prime value. 



After making many groupings of the formations and testing many 

 tentative correlations, it has seemed that the most logical arrange- 

 ment is secured if the principal weight is given to the graptolites. It 

 appears that these organisms had spread very much more rapidly than 

 any of the other animals, except for a few thin-shelled brachiopods and 

 trilobites which may have been dispersed by the same agency as the 

 graptolites. When relatively short distances are in question, it seems 

 that the bottom dwelling animals were able to keep pace in their 

 migrations with the graptolites, at least sufficiently closely so that we 

 detect no difference in the geological record, but when long distances 

 are traversed, the bottom animals lag very considerably behind. 

 Striking cases are those of Shumardia, which preceded the first Tetra- 

 graptus fauna in Scandinavia, and reached America only with the last 

 Tetragraptus and the first Diplograptus, and of Echinosphaerites, 

 which preceded the Xemagrapfus gracilis fauna in Scandinavia and 

 followed it in America. 



