Hyatt.] 302 



and portions of its waters might have come under the control of the 

 trade winds of the Atlantic Ocean, and been forced to the eastern 

 coast of America. There exists some palaeontological evidence which 

 could be adduced to support this view of the passage of the equato- 

 rial current from the Indian Ocean across the waters which held the 

 life of the Tertiary period now fossil in the beds of Southern Europe; 

 but an examination into this question would demand special consider- 

 ations, not coming within the scope of this paper. 



It is with regard to the period at which the northern half of the 

 equatorial current was broken by the upheaval of the intertropical 

 portion of the American continents, that we have probably the least 

 satisfactory evidence. No palasontological evidence tending to prove 

 the former connection of the Atlantic and Pacific Oceans in intertropical 

 re^-ions has yet been published, so far as is known to the author. But 

 we may derive some light from a consideration of the magnitude of 

 the elevations which have taken place along the great Avestern axis of the 

 American Continents since the beginning of the Tertiary period. To 

 the north and south of the Isthmus connecting the continents, we 

 have evidences of elevation amounting to from three thousand to six 

 thousand feet or upwards. The whole northern coast of South Amer- 

 ica as well, gives evidence of great elevation since the Eocene 

 period. If we examine the elevation of the existing land of the Isth- 

 mus, and compare it with the magnitude of the uplift at other 

 points in the same range, we are forced to the conclusion that if any 

 thin"- like the same rate of elevation was effected in Central America, 

 the emergence of this region could not have accomplished the disrup- 

 tion of the equatorial current at this point, until the Tertiary period 

 had been somewhat advanced. 



The foregoing considerations render it probable that the great 

 meridional streams of the Atlantic and Pacific Oceans, with their 

 great effects on the distribution of life and of sedimentary deposits, 

 are phenomena which have most likely been in existence, only since 

 the beginning of the Tertiary period. 



Mr. A. Hyatt made a communication npon the agreement 

 between the different periods in the life of the individual 

 shell, and the collective life of the Tetrabranchiate Cephalo- 

 pods. He showed that the aberrant genera beginning the 

 life of the Nautiloids in the Palaeozoic Age, and the aber- 

 rant genera terminating the existence of the Ammonoids in 

 the Cretaceous Period, are morphologically similar to the 

 youngest period and the period of decay of the individual ; 

 the intermediate normal forms agreeing in a similar manner 



