530 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 112 



species were almost invariably confined within circumscribed and 

 isolated geographical regions. 



Investigation of the hydrography of these areas revealed that only 

 very slight temperature variation occurred in them, and Fage sug- 

 gested that this factor is the dominating one restricting the spread of 

 the Lophogaster population. He advanced an interesting Irypothesis 

 to explain the sporadic distribution of the members of the genus and 

 the restriction of each to its own isolated locality in the world today. 

 He suggested that these isolated species represent the survivors from 

 an ancestral form which in earlier epochs was widely spread throughout 

 the world. Owing to an inability to tolerate changes of temperature, 

 this early form died out in those regions in which, owing to geological 

 changes, considerable variations in temperature occurred. Only in 

 isolated areas where the tsmperature remained relatively stable did 

 remnants of the old stock survive and in their enforced isolation evolve 

 those combinations of small differences whereby they may be separated 

 into species today. 



There is much to commend this hypothesis. Certainly the geo- 

 graphical distribution is a most valuable guide for the identification 

 of species, though one or two cases are known of more than one species 

 occurring in the same area. The correlation of distribution with areas 

 which have a small range of temperature is most striking. Where 

 there is a horizontal spread in the range of a species, this spread is 

 usually associated with the flow of warm or cooler ocean currents. 



At the present time the genus includes 15 species, but the present 

 research has convinced me that 2 of them are synonymous with 2 

 previously described species. The 13 remaining species are distributed 

 as follows : 



Atlantic and Western Indian Oceans 



1. L. lypicus M. Sars (1857): coastal waters of western Europe from 



southern Norway to the Bay of Biscay, west of Ireland; 

 Mediterranean. 



2. L. subglaber Hansen (1927): south of Spain off Cadiz and Gibraltar. 



3. L. spinosus Ortmann (1906): tropical midsouth Atlantic and off 



Puerto Rico, the Bahamas, and between the Bahamas and the 

 coasts of the Carolinas. These regions have the highest tempera- 

 tures in the Atlantic of the Northern Hemisphere. 



4. L. longirostris Faxon (1896, = L. americanus W. M. Tattersall, 



1951): Gulf of Mexico, West Indies, and along the path of the 

 Gulf Stream to the waters off Massachusetts. 



5. L. challengeri Page (1941): coastal waters off South Africa from Cape 



Town to Angola in the path of the Benguela Current. 



6. L. rotundatus Illig (1930): off the Saya da Malha Bank, Central 



Arabian Sea; Straits of Zanzibar and off the coast of southeast 

 Africa to Durban and Port Elizabeth along the path of the warm 

 southward flowing Mozambique Current. 



7. L. affinis Colosi (1930): Red Sea, northern and central region. 



8. L. crythraeus Colosi (1930) : south of the Red Sea near Bab el Mandeb. 



