366 



SGIENGE. 



[N. S. Vol. III. No. 62. 



Especially the latter process was a relatively 

 recent one, but it existed long enough to pro- 

 duce differences in the two faunas. The former 

 conditions on the other hand can be recognized 

 very frequently in the jiresent distribution. 



Probably the abyssal life-zone was formerly 

 not so extensive as to-day. It is probable also 

 that during periods before a decrease of tem- 

 perature at the poles the conditions of tem- 

 perature were quite different from our present 

 ones. The abyssal may therefore be of rela- 

 tively recent date. 



The pelagial must be very old, certainly as 

 old as the litoral and continental. It was quite 

 continuous in former times. With the climatic 

 diflFerentiation of the poles a corresponding dif- 

 ferentiation of the pelagial took place ; the 

 circumtropical belt remained for some time con- 

 tinuous. The separation into the Atlantic and 

 Pacific region was produced by the Isthmus of 

 Panama. This differentiation is very recent, 

 and the pelagial of the two new regions are ex- 

 ceedingly similar. 



The chapter concludes with some remarks on 

 the biological (biocoenotic) changes of the earth 

 history. 



The sixth chapter is devoted to the Bionomy 

 and geographical distribution of the Decapoda. 

 Dr. Ortmanu is an authority on this group of 

 crustaceans. An exhaustive essay on the geo- 

 graphical distribution, he says, is at present not 

 possible, since some of the smaller groups have 

 not been sufficiently studied ; but he thinks it 

 feasible to give a general view of the bearing 

 of the Decapoda on the points discussed in the 

 former chapters. 



In many ways, he says, the Decapoda are a 

 typical group for zoogeographical studies. Here 

 all the possibilities of bionomic conditions are 

 found, and they therefore constitute an especi- 

 ally good example of distribution. The an- 

 cestors of the Decapoda were nectonic animals, 

 which were dependent on the bottom, could 

 therefore only cross in the litoral or abyssal ; 

 we may exclude the abyssal, and have to con- 

 sider the first Decapoda as nectonic litoral forms. 



By far the greatest number of the Decapoda 

 is litoral; a great number, however, lives also 

 in the abyssal, and partially quite distinct syste- 

 matic groups have their main distribution in this 



region. The most important families which 

 are exclusively abyssal are: the Acanthephy- 

 ridee and Nematocarcinidse, the Glyphocran- 

 gonidse, Ergonidaj and Thaumastochelidse. All 

 these as compared with their nearest relatives are 

 more primitive groups, which must have immi- 

 grated into the abyssal, already in remote ages 

 undergoing thereby but slight modification. 

 On the other hand, the abyssal received more 

 forms in later times. These are groups, the 

 nearest relatives of which, often belonging to 

 the same genus, are still found in the litoral. 

 It is interesting to note that some of the latter, 

 for instance the Craugonidaj and Lithodidse, 

 perhaps also the Pandalidse, point quite defi- 

 nitely to the polar litoral. The opinion that 

 the abyssal is especially characterized by primi- 

 tive forms is not correct ; both the litoral and 

 fluvial possess primitive forms. 



The only Decapods which have been adapted 

 to the Pelagial as true planctonic Crustaceans, 

 are the Sergestidte, which represent a highly de- 

 veloped branch of the nectonic-litoral Penaids; 

 they seem therefore to be of comparatively 

 great age. The other forms of the high sea, 

 the inhabitants of the Sargassum are quite iso- 

 lated and belong to members of quite different 

 groups. Forms like Varuna and the Plagusiinse 

 can hardly be considered pelagic ; since they 

 often live on the coasts, and are perhaps driven 

 out on swimming objects more frequently to 

 the open sea, as a result of their mode of life. 



The most important fluviatile Decapods are 

 the Atyidte, the group Palsemon and Bithynis 

 among the Patemonida;, the Potamobiidfe and 

 Parastacida3; the Aegieidte (represented by a sin- 

 gle form), the Thelphusidse and Sesarminae. The 

 two last are partially adapted to subterranean 

 life. These groups are of very different age, and 

 their immigration into the fresh water has 

 taken place at very different times ; hence the 

 geographical distribution of each of these groups 

 must be studied separately. 



True continental forms, perhaps sometimes 

 still frequenting the sea, are the Coenobitidaj 

 and Gecarcinidas ; both are moi'phologically re- 

 cent and specialized groups. 



The Decapods of the litoral and abyssal in- 

 habit all the different facies of these zones. 



On the succeeding pages the characteristic 



