10 Pratt, Marine Fauna of the Falkland Islands. 



water, but only occurs in the tropics in deep water. He 

 suggests that many forms which have been recorded from 

 northern and southern seas, but not from the tropics, may 

 occur in the tropics in deep water and have consequently 

 escaped capture. 



Ortmann further maintains that the migration of 

 forms may take place from the northern hemisphere 

 through the tropics, to the southern hemisphere along the 

 west coasts of America and Africa, because of the compara- 

 tive low temperature in tropical regions along these coasts. 

 This is confirmed in the Decapods, and the reverse — i.e. 

 the migration of forms from the southern to the northern 

 hemisphere — in the case of the Isopod Serolis. 



Before discussing the bearings of these two theories, 

 it would be useful to consider the distribution of the 

 genera in the collection. 



The genera represented are : — 

 Cheilostomata. 



Beaniuy Cellaria^ Cellepora^ Cribrilina^ Lepralia, Mem- 

 branipora, Micropora^ Microporella, Mucronella^ Schizo- 

 porella, Smittia, Porella ; and, among the Ctenostomata, 

 Bowerbankia. 



All the species of Beania, with one exception {B. 

 hirtissima), occur north and south of the tropics in 

 temperate regions, but not within the tropics. B. hirtissima 

 has been recorded from Cape Verde Islands, which lie 

 within the tropics. All the species, except the British B. 

 mirabilis, occur in shallow water. 



The genus Cellar ia is cosmopolitan. It occurs in deep 

 and shallow water in the temperate zone. The depths at 

 which it occurs in the tropics I have not been able to 

 ascertain. It occurs in the fossil tertiary strata. 



Cellepcra. The Challenger obtained 30 or 3 1 species 



