The Ccelentera in Relation to Geological Zones. 61 



Glossograpt2is hincksii, Didyinograjotus superstes^ Cceno- 

 graptus gracilis, C. perfenuis, Dicellograptus sextans^ 

 D. zic-zac, D. divaricatus, Diplograptus mucronatus, 

 D. hhnuci'onatus. 



Below these, other graptolite faunas occur elsewhere ; and 

 others, again, occur above ; but with these we have at present 

 no particular concern. 



The question remaining to be answered after the fore- 

 going facts have been duly considered is this : — 



If, as is believed to be the case, the sea-bottom in the 

 area now represented by Moffat presented nearly uniform 

 conditions throughout the entire period during which these 

 graptolite-bearing mudstones were being formed, how has 

 it happened that so many and important changes in the 

 animal life of the period were brought about ? There is no 

 escaping the conclusion that this small thickness of sedi- 

 ments represents the whole of the deposits formed there 

 throughout a period of immense length, during which im- 

 portant changes of geographical conditions, and equally 

 important changes in the organic world, are known to 

 have taken place elsewhere. 



It seems to be certain that the whole of the life of the 

 graptolites cannot have been passed at or near the bottom of 

 the sea. Had this been the case, it is inconceivable that 

 repeated changes, such as occur in the fauna, could have 

 followed each other so closely in an area where the condi- 

 tions affecting life remained uniform. 



If, on the other hand, we may assume that graptolites 

 resembled most of the Ccelentera in passing the earlier stages 

 of their existence at or near the surface, as free- swimming 

 fry, it may be possible to find a solution of what has long 

 remained an enigma to geologists and palaeontologists alike. 

 We have but to assume that the fry of certain species of 

 graptolites were vitally affected by any lowering of the tem- 

 perature of the sea-water by which they were surrounded, 

 and that the temperature of the ocean-surface was liable to 

 variations, such as might be caused by the changes in 

 direction of the oceanic surface currents. Such of the fry 



