392 ORTMANN — DISTRIBUTION OF DECAPODS [Aprils, 



the original presence of crabs in Africa at about the middle of the 

 Cretaceous would explain the fact that no crayfishes are found on 

 this continent ; but, on the other hand, there is the possibility that 

 crayfishes once existed there, but have become extinct on account 

 of the increase of crabs in this country. 



Then, again, after the crayfishes had, in Upper Cretaceous times, 

 occupied western North America and Mexico, they met here with 

 the crabs which came from the south, and their farther advance 

 was checked by this biocoenotic barrier. 



The question remains, why did the crabs not advance beyond 

 their present (and old) boundaries in China, Australia and Mexico? 

 If it is correct that the existence of crabs forms a barrier to the ex- 

 tension of the crayfishes, the opposite cannot be the case. The 

 presence of crayfishes would not put a stop to a farther dispersal of 

 crabs. But here, I think, we have to deal with climatic barriers. 

 All freshwater crabs are truly tropical animals, entering in only a 

 few cases subtropical countries, but never temperate or cold regions, 

 and thus it seems that the northern boundaries of the Potamonidce 

 in China and Mexico and the southern in Australia are due to the 

 climate of these respective parts. The same seems to be true in 

 Europe, Western Asia and in Bolivia, where the northern, resp. 

 southern boundaries are apparently given, in a large part, by some 

 features of the climate. 



It will be noticed that in applying this principle to the past dis- 

 tribution of the crabs it is necessary to assume the existence, in 

 earlier Tertiary and even Pretertiary times, of climatic differences 

 on the continents, although we do not believe in a climatic differen- 

 tiation of the oceans of the Mesozoic period. But this is entirely 

 in keeping with our opinion expressed in a previous paper. 1 And, 

 further, I do not mean to say that the present climatic boundaries 

 of the crabs are identical to those of former times. On the con- 

 trary, it is quite possible, for instance, that in China the crabs 

 formerly extended farther north, and in Europe we know positively 

 that the European species did so in Diluvial times, reaching as far 

 as Hungary, where it does not now live (see above, p. 376, foot- 

 note). The southern boundary of the crabs in Australia, however, 

 seems to be original and has not retreated equatorward, since these 



1 Ortmann, A. E., " An Examination ... of Climatic Zones in Jurassic 

 Times," in Amer. Journ. S<-i., Vol. 1, 1896, p. 270, foUnote. 



