74 EDITH M. PRATT, B SC. 



common descent. He maintains that an exchange of both polar forms 

 can take place through the deep sea, on the ground that, among 

 Crustacea, the Cosmopolitan genus Ponlophilus shows a decided 

 tendency to retire into deep 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 w r est coast of America and Africa, 

 because of the comparative 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. 



Beania, Cellaria, Cellepora, Cribrilina, Lepralia, Membranipora, 

 Micropora, Microporella, Mucronella , Schizoporella, Smittia, Porella ; 

 and, among the Ctenostomata, Boiverbankia. 



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. 

 mirahilis, occur in shallow water. 



The genus Cellaria 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. 



Cellepora. The Challenger obtained 30 or 31 species' of this genus, 

 of which the North Atlantic yielded three, from depths varying from 

 51-400 fath. 



The South Atlantic yielded 9 species, 5-600 fath. 

 ,, Kerguelen region ,, 6 ,, 20-500 ,, 

 ,, Australian ,, ,, 12 „ 2-40 ,, 



