Manchester Memoirs, Vol. xlv. (iQor). No. 14. 3 



between extra-tropical forms does not indicate a common 

 descent, but is in many cases a "secondary reappearance." 

 He holds that an interchange of polar forms may take 

 place through the deep sea, on the ground that among 

 the Crustacea the cosmo)Jolitan genus PontopJiilus shows a 

 tendency to retire into deep water, and only occurs in the 

 tropics in the deep sea. He suggests that many bipolar 

 forms may occur in the tropics in deep water and have 

 thus escaped extermination. 



This point, however, is by no means proved, for in the 

 case of many littoral bipolar forms we have no evidence 

 that they ever retire into deep water. 



In explanation of the distribution of such forms, he 

 maintains that an interchange of supposed bipolar forms 

 may take place through the tropics along the western 

 shores of America, where, owing to cold ocean currents, 

 etc., the temperature of the tropical waters is lowered. 

 This would explain the occurrence of some, but not all, 

 similar forms in the north and south temperate Pacific. 



On the ground that a variety of the European Mata 

 squinado {M. squinado capenszs) has been taken at the 

 Cape of Good Hope, Ortmann further maintains that 

 a similar interpassage of forms takes place along the 

 western shores of Africa. This, however, is by no means 

 proved, and it is within the province of this paper to show 

 that a variety of a northern European species {Goniada 

 norvegzca)* may occur in the south Atlantic (Falkland 

 Islands), while the genus is represented in the tropics, i.e., 

 along the western shores of Africa, by several distinct and 

 modified species. 



That an interchange of extra-tropical forms takes 

 place along the western shores of America and Africa is 

 only proved for such forms as have actually been taken in 



* Manchester Memoirs, Vol. 45, No. 13, p. 3. 



