194 , PROCEEDINGS OF THE ACADEMY OF [1895. 



separated from the cooler parts of the southern by the broad belt of 

 the warm circumtropical seas. This zone, however, is not a con- 

 tinuous one, but is interrupted within the litoral on two tracts, on the 

 western coasts of Africa and America. At these two localities there 

 are two causes producing a lower temperature of the litoral seas than is 

 usual in the tropics. On the one hand, there are cold currents run- 

 ning from the southern cold seas along both shores northward as far 

 as the equator and even beyond; on the other hand, on these coasts 

 arises cold water from the sea-bottom, the equatorial currents directed 

 from the coast to the west carrying away the surface water. A 

 cooling of the litoral waters of the w r est coasts of Africa and America 

 is thus produced, and although the most superficial layers of water 

 may be warmed by the sun, in greater depths within the litoral 

 there may prevail a low temperature. Thus, on the west coasts of 

 Africa and America, it may be possible that northern litoral forms 

 penetrate into the tropics and beyond, and may reach the litoral of 

 the cooler antartic hemisphere. The presence of Crangon capensis 

 and C. antarcticus may be thus explained. 



By adaption to a cooler temperature a large number of Cran- 

 gonidse are able to descend to greater depths, 16 and by this habit 

 they may enter and cross the tropics in the deep sea. The species 

 adapted to the greater depths show, as usually in deep sea animals, 

 a very large horizontal range, and, therefore, they can reach 

 the southern hemisphere, while a re- ascending into the litoral 

 of the antarctic regions is possible. We know of the genus Pon- 

 tophilus, which is the only one containing true deep sea species of wide 

 distribution, two species in southern Australia and New Zealand, the 

 presence of which is probably due to this cause. 



Other barriers against the distribution of the species of Cran- 

 gonidse are of a topographical character. At first, the great conti- 

 nents of northern Eurasia and North America cause a complete 

 separation of the northern temperate parts of the Atlantic and 

 Pacific, and, therefore, these oceans contain distinct species. 

 Farther, the Crangonidge, living mostly benthonic, can not pass over 

 great oceans, and, accordingly, the eastern and western shores both 



16 The cold temperature is the main cause favoring immigration in the 

 deep seas, and in the arctics the deep sea and the litoral are closely connected. 

 See Monaco, Zur Erforschung der Meere, etc., translated into German by 

 Marenzeller, Wien, 1S91, p. 135, and Pfeffer, Versuch über der erdgeschichtliche 

 Entwickelung der jetzigen Verbreitungsverhältnisse unserer Tierwelt, Hamburg, 

 1891. 



