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1419 



TERTIARY FAUNA OF FLORIDA 



sentatives of the group, I am persuaded that America is the centre from which 

 the group has been distributed. In the fossil state it is characteristic of the 

 Eocene ; it was named by Conrad " the finger-post of the Eocene," and it does 

 not occur (except as a fossile remanie) in OHgocene or later strata. 



The correlation of the different horizons of our Eocene with those of France 

 and western Europe is based, not on the presence of identical species, which 

 (excluding the boreal fauna) is hardly to be expected, but on the arrival of 

 the fauna as a whole at an analogous stage of evolution. As climate developed 

 differences in different parts of the globe, the dynamic effect of these differ- 

 ences on the faunas exposed to them would make itself felt in differentiation 

 of species. Therefore even if identical generic groups occur in widely sepa- 

 rated Tertiary faunas we should expect to find parallel, not identical, groups 

 of species. This is what in reality we do find, and also that each fauna, as a 

 rule, is clearly genetically linked with its predecessor in the same region. Only 

 in cases of an abrupt change of conditions, especially temperature, do we find 

 a break in the genetic series such as is offered by the succession of the Chesa- 

 peake cold-water Miocene on our southeastern coast to the warm-water Oligo- 

 cene which preceded it. In minor changes, however, we occasionally find a 

 resistant molluscan type which persists with only minor modifications and 

 spreads to the uttermost parts of the earth, as in Paleozoic times. In the 

 Eocene the V. planicosta type is one of these ; so is l\ imbricata and Natica 

 crassatina of Lamarck. 



We find in our Eocene and that of the Paris basin the following parallel 

 forms in this group : 



America. France. 



V. alticostata, V. imhrkata, 



V. dscia, V. planicosta, 



V. marylandica, V. pectuncularis, 



V. rotunda, V. acuticostata, 



V. tridentata, V. decussata. 



Allowing for differences of generic types, such parallels might be increased 

 almost indefinitely, and it is this fact, and not the presence of identical species, 

 which justifies the correlation of the different horizons. 



The distinctions between the Parisian and the American shell passing under 

 the same name were pointed out by Rogers in 1839,* who also fully illustrated 



* Trans. Am. Phil. Soc, N. S., vi., p. 374, pi- xxix., figs. 2 a-c. 



