272 ORTMANN— DISTRIBUTION OF DECAPODS [Aprils, 



those abnormal cases which resemble each other. Thus we gain 

 certain general views as to ancient geography, and we are finally 

 enabled to trace the distribution of land and water, of climatic con- 

 ditions and the like in the geological past. 



Most prominent among the groups of animals that are available 

 for these investigations are the Mammals^ and they have actually 

 been used for just this purpose by various authors (Doederlein, 

 Zittel, Lydekker, Scharff, Osborn). The palaeontological material 

 within this group is the most complete of all. But there is one im- 

 portant drawback : since the history of the Mammals hardly goes 

 back beyond Tertiary times, at any rate since the palseontological 

 record of this group is more or less complete only within the Ter- 

 tiary, we can only draw conclusions from them as to the geographi- 

 cal conditions of this period, while we have to refrain from an 

 investigation of those of the Mesozolc times. 



This is a very different matter with the land and freshwater Mol- 

 lusks. According to what we know, it is apparent that many of 

 these forms can be traced back to Mesozoic times, sometimes even 

 to Palaeozoic, and, indeed, it is this group of animals that has fur- 

 nished the material for the studies of von Ihering, Piisbry, Hedley, 

 Kobelt, and we are to expect that further investigation in this direc- 

 tion may yield interesting results. 



Other groups have also been used. Von Ihering introduced the 

 study oi Ants, and there may be other promising groups among the 

 Insects (for instance Spiders). But since the majority of the Insects 

 possess unusual means of dispersal (power of flight) that are apt to 

 obscure the original conditions of distribution. Insects in general 

 are not well adapted to this kind of research. Of other animals the 

 Earthwofms have been studied in this respect (by Beddard), and 

 of the Vertebrates, Reptiles, Amphibians, and freshwater Fishes are 

 very likely to prove good objects, since their history in many cases 

 goes back to the beginning of the Mesozoic or even to the Palaeo- 

 zoic time. 



In the following treatise I wish to call special attention to certain 

 groups of Decapod Crustaceans that live in fresh water. In part 

 these have been discussed previously by other writers as well as by 

 myself, but it is worth while to go more into detail, since we shall 

 find them very interesting in this respect. 



