i894. MIGRATIONS OF EARLIEST FORMS. 421 



It is a tempting field for speculation to enquire why this should 

 be the case. It has been suggested that Olcnellus was a form pecu- 

 liarly adapted for living in shallow water and along shore-lines ; 

 Pavadoxides, on the other hand, is a much larger form, and would 

 require somewhat deeper and more open water. It is the dominant 

 form in the Middle Cambrian, and attained to a size of over twenty 

 inches in length, being the largest trilobite known. When the 

 Middle Cambrian rocks were deposited, a considerable depression 

 had taken place, and though the depth was not great, the sediments 

 must have been deposited in a fairly open sea. Olenns, the typical 

 genus of the Upper Cambrian, was of small size, and has the appear- 

 ance of a degraded form of Pavadoxides. 



When I ventured to suggest that the " home of the earliest form 



of life seems to have been somewhere towards the south-west, and 



possibly not far from the equator," 7 I was, of course, referring 



specially to the North Atlantic Ocean. The other oceans would 



doubtless be equally tenanted by life, and would form centres of 



dispersion. In these early oceans there would, of necessity, be 



forms of life suitable to the varying depths ; but it is not to be 



expected that representatives of all these could occur in any of the 



earliest faunas with which we are acquainted. When deeper water 



faunas are found in the succession, there, as in the littoral faunas of 



the early Cambrian, abundant appropriate organisms suddenly 



appear, showing that they had occupied areas favourable to. their 



existence contemporaneously with the earlier faunas entombed in 



shallow-water deposits. As an instance of this we may mention 



the sudden incoming of the very rich graptolite fauna of the Arenig 



rocks. 



Henry Hicks. 



^ Quart. Journ. Gcol. Soc, vol. xxxi., p. 555. 



