280 KEY TO TERRESTRIAL HISTORY 



Thus the correlation of the Jurassic beds of Cutch with 

 those of Europe depends entirely on the Ammonites, 

 it could never have been established by a study of the 

 Corals or Brachipods, for instance. The Ammonites 

 possessed peculiar chambered shells, constructed on the 

 same principle as those of the modern Nautilus or Spirula. 



The Nautilus lives at considerable depths in the sea, 

 and is rarely seen at the surface, yet its shells are found 

 widely scattered over the shores of the Pacific Ocean. 

 The numerous specimens to be found in museums or 

 adorning old-fashioned collections of " curiosities " have 

 almost all been picked up off the beach. The explanation 

 of this lies in the fact that after the death and decay of 

 the animal, the shell, owing to its water-tight compart- 

 ments being filled with air, rises to the surface of the 

 sea, and is then blown by winds or drifted by currents 

 till it strands on some shelving shore. 



The little Spirula is, perhaps, even a more striking 

 illustration of this method of distribution ; it is very 

 rare in the flesh, not more than half a dozen speci- 

 mens have ever been seen alive, and it is confined 

 during life to tropical seas, yet its shell is one of the 

 common objects of the seashore. It is found floating 

 in mid-ocean, and is washed up on almost all shores 

 of the world, finding its way even to the northern coast 

 of Scotland. 



From these facts Professor "Walther has been led to 

 suggest that the wide distribution of single species of fossil 

 Ammonites may be accounted for in a similar manner ; 

 they may have lived remote from the spots where 

 they are now found fossil, and have been dispersed far 

 and near as dead shells by wind drift and ocean currents. 

 If so the difficulties of the biologist would be met, but 

 it is doubtful how far the explanation can be regarded 

 as general : Ammonites, like modern cuttle-fish, appear 



