1895.] NATURAL SCIENCES OF PHILADELPHIA. 195 



of the Pacific and Atlantic are inhabited by different species. Only 

 the true arctic species being able to live along the most northern 

 shores of America and Asia show a circumpolar distribution, no 

 topographic barriers of the kind mentioned there being present. 

 The species not living in the arctic, but in the boreal 17 seas, are re- 

 stricted by such topographic barriers. 



It is evident, therefore, that the means of dispersal of the 

 Crangonidse, except Pontocaris and a few species of Pontophilus, do 

 not act against the climatic barriers and mostly also not against the 

 topographic. Only a few known species are present on both shores 

 of the Atlantic: Pontophilus norvegicus (100-150 fath.) and 

 Sabinea sarsi (60-150 fath.). Whether these species can pass over 

 the barrier formed by the northern Atlantic as adult animals or as 

 larva 1 , or whether this distribution is due to other causes, we can not 

 say at present. 



On the shores of the Atlantic and Pacific very nearly allied species 

 are sometimes found. These must be derived from common ancestors 

 living when the arctic ocean was not as cold as at present, and when 

 a circumpolar connection was present for these species as in the case 

 of the circumpolar forms now living. Later these species retreated 

 more southward, and by the topographic separation of the range, 

 the morphological characters could change, and distinct forms could 

 develop. 



The geographical distribution of the Crangonidse is a very char- 

 acteristic one and important as limiting the northern zoo-geographical 

 regions of the litoral. Apart from a few species living in the tropics, 

 in the antarctic and in the deep sea, the family of Crangonidre char- 

 acterizes the northern circumpolar region, as defined by me formerly. 18 

 This region is characterized by the genera Nectocrangon and Sabinea. 

 Among the northern species we can distinguish true arctic species 

 showing a circumpolar range, especially Crangon salebrosus, boreas, 

 Nectocrangon lar, Sabinea septemcarinata, and boreal species. The 

 latter are not circumpolar, but more restricted. Crangon crangon 

 affinis is restricted to the Pacific, Pontophilus norvegicus and Sabinea 

 sarsi to the Atlantic. The other species are more localized and char- 

 acterize each a separate local fauna, and we can distinguish a Japan- 

 ese fauna, a fauna of the Berings Sea, a fauna of the western coast of 



17 Regarding the distribution of "arctic" and "boreal" seas see Pfeffer, 

 Versuch, etc., 1891. 



18 Jenaiscbe Denkschriften, VIII, 1804, p. 78. 



