1888.] NATURAL SCIENCES OF PHILADELPHIA. 287 



This view has been advocated by the well-known conchologist Dr, 

 W. Kohelt' and by others. 



But although this theory explains many anomalies in the distribu- 

 tion of molluscs, I must freely confess that the objections to it seem 

 to me almost insurmountable. The recent work of the Challenger, 

 Blake, and other deep-sea explorations, all tend to confirm the view 

 held by Guyot, Dana, Agassiz and others, that the great oceanic 

 basins, practically as they exist to-day, are of great antiquity; and 

 render the existence of a former Atlantic continent with any con- 

 siderable ^Yestern extension, highly improbable. 



A view more in accordance with the facts* with which we are at 

 present acquainted, seems (o me to be the following: It is a well 

 ascertained truth that until toward the close of the Miocene, large 

 portions of Nothern Africa as well as Europe were submerged ; and 

 it appears probable that the westward flowing Equatorial current of 

 the Indian Ocean extended across northern Africa, and united with 

 the Atlantic northern equatorial current, which now flows westward 

 from northern Africa, through the Antilles into the Gulf of Mexico.. 

 This current would afford a means of transport not only for the free 

 swimming embryos of marine molluscs, (and there are not a few 

 forms both of gasteropods and pelecypods, common to the Mediter- 

 ranean and Gulf Provinces,) Init also, through the agency of floating 

 materials, trees, etc., swept from rivers, land mollusks may have 

 been transported across the Atlantic, just as they have been carried 

 by the Gulf Stream from the West Indies to the outlying island of 

 Bernuida," a distance of over 700 miles. 



A further development of the same idea explains certain peculiari- 

 ties in the distribution of species common to the Pacific and the 

 Gulf of Mexico. The presence of Miocene and Pliocene deposits 

 render it certain that there was communication between the Gulf 

 and the Pacific across the isthmus of Panama as late as the Pliocene. 

 And a portion of tlie equatorial current probably swept directly 

 through to the Pacific. Thus it is likely that those forms common 

 to both sides of the isthmus, will prove to be of Atlantic origin, and 

 to have been distributed westward. 



The indigenous Bermudan mollusc-fauna, marine as well as terres- 

 trial, has undoubtedly been derived wholly from the West Indies. 



1 Nachricht.sblatt d. deutschen Malak. Gesell., J8S7, p. J47. 



2 See Darwin, Origin of Species, Gth ed., p. 353. Also a paper by Mr. C. T. 

 Simpson, On tlie Distribution of Land and Fresh-water Shells in the Tropics, 

 Conch. Ex. ii, p. 37, 50. 



