306 BULLETIN OF THE 



distribution at different geological epochs. The greater the bathy- 

 metrical range of littoral species, the longer will such species remain 

 unaffected, while deep-sea species may early become isolated and re- 

 main as outliers as it were, — mementos of a former condition of cur- 

 rents, or even of a previous geological period. The careful analysis of 

 the Fauna of a given point, its comparison with other Fauna}, and accu- 

 rate bathy metrical data, would go far towards reconstructing the Natural 

 History of the sea in former ages, and showing its relation to the 

 present and past times. 



The representative species of Echini, Echinocardium, Psammechinus, 

 Schizaster, in the Arctic and Antarctic boreal zones would be con- 

 sidered as the living representatives of a cosmopolitan Fauna existing 

 at the time when the great equatorial current flowed unbroken round 

 the globe, sending branches north and south along Eastern North and 

 South America, along Eastern Japan and Australia, and the eastern 

 coast of Africa; while the tropical species of the genera Diadema, Cly- 

 peaster, Echinoneus, Echinolampas, &c, existing at that time, had a 

 more limited equatorial geographical distribution. The subsequent 

 period of isolation of Atlantic and Pacific currents is shown by the 

 existence of truly Atlantic and Pacific species ; while as we go down in 

 depth we go back also in time, and find at first representatives of 

 the genera found in our Tertiaries, while at greater depth the species 

 are representatives of genera found in the Cretaceous. A more de- 

 tailed comparison than can be given here of the Caribbean Fauna, with 

 the fossils of the tertiary and cretaceous deposits of our coasts, would be 

 most interesting ; but unfortunately the materials thus far collected are 

 too fragmentary, and we must await a careful geological survey, accom- 

 panied by deep dredgings of a considerable extent of coast, before we 

 shall have- the data needed to follow up the important results to be 

 gained in this way for palaeontology and geography, of which our 

 present incomplete materials give us such an interesting glimpse. 



