Reviews — Ball's Tertiary Fauna of Florida. 137 



'between free -swimming and sedentary forms, or rather between 

 sedentary forms and forms that go through a floating or free- 

 swimming stage lasting some time ? On looking through Professor 

 Dall's monograph we are disappointed to find, though it is no fault 

 of his, that practically the whole of the molluscan faunas described 

 consist of sedentary forms. If we could compare the pteropods, 

 Janthina, ship- worms, barnacles, sharks, and such like on the two 

 sides of the Atlantic, we should probably discover the true 'Atlantic' 

 fauna for each period, which would leave no doubt as to the exact 

 correlation. At present, for instance, we only know the American 

 and European Eocene faunas of the shallow seas, we are only 

 slightly acquainted with the true Atlantic Eocene fauna. In time 

 these gaps will be filled up, and we shall be able to correlate with 

 greater certainty. 



The careful and sober account of the physical changes in the 

 Antillean region, given in the " Discussion of the Geology " 

 (pp. 1541-1620), needs close study and cannot easily be condensed ; 

 it is in striking contrast with much of the wild speculation that has 

 been rife. The physical and climatic changes are traced step by 

 step, evidence being given for each statement. In Eocene times 

 the two oceans were separate. The Oligocene deposits of Florida 

 are of enormous thickness, and there is evidence of a connection 

 between the Atlantic and the Pacific. In Miocene times the two 

 Americas became again connected, and the fauna of the Gulf coast 

 changes completely. " The change was not only in the species and 

 prevalent genera of the fauna, but a change from a subtropical to 

 a cool temperate association of animals. Previously, since the 

 beginning of the Eocene, on the Gulf coast the assemblage of 

 genera in the successive faunas uniformly indicates a warm or 



subtropical temperature of water With the incursion 



of the colder water the change becomes complete. Not only do 

 northern animals compose the fauna, but the southern ones are 

 ■driven out, some of them surviving in the Antilles to return later. 

 Some change along the northern coast permitted an inshore cold 

 current to penetrate the Gulf. . ..." A cool Miocene sea in 

 the Gulf of Mexico is a phenomenon which will have to be taken 

 into account by the student of geographical distribution. In con- 

 junction with a temperate Miocene climate in the Arctic regions 

 it may help to explain the occurrence of closely allied land-animals 

 and plants on the two sides of the Atlantic, and in the northern and 

 southern hemispheres. 



As to subsequent changes. Professor Dall writes : "I concur with 

 Hill in the belief that, whatever changes of level may have taken 

 place since, no discontinuity of the link between North and South 

 America from the Miocene to the present time is probable, and 

 certainly none amounting to a free communication between the 

 two oceans." 



Towards the close of the Miocene period Florida became united 

 to the continent, and the influx of cold water into the Gulf of 

 Mexico ceased. Gradually the temperature rose, and the exiled 



