CH. Ill] DISTRIBUTION OF SEROUS. 175 



They are most prevalent on the coasts of Patagonia. A 

 few, but very few, species range widely through the 

 antarctic region; thus S. septemcarinata extends from 

 S. Georgia to Kerguelen. The Australian species, with 

 the exception of Serolis. minuta, form a well-marked 

 assemblage to which perhaps a different generic name 

 ought to have been given ; these species differ from the 

 typical species of the genus in the fact that the dorsal 

 part of the sixth thoracic segment is absolute in the 

 middle line ; there are also other smaller particulars in 

 which these Australian species diverge from the normal 

 structure of the genus. 



Here we have a family of curiously restricted range ; 

 it is exclusively antarctic, not getting so far north as the 

 equator. It may be surmised that the genus came into 

 being in the antarctic region; whence conditions of 

 temperature did not allow it to migrate. But a difficulty 

 is presented by the wide range within this area of species 

 which being addicted to shallow water would, find some 

 hindrances in crossing the thousands of miles of deep 

 water which part Patagonia from the Crozets. Possibly 

 this difficulty is removed by the supposition that in former 

 times the antarctic continent extended further north than 

 it does now, thus enabling the species to wander far 

 without altering greatly the surrounding conditions. The 

 marked differences which distinguish the Australian 

 species from their more southern allies are so far a proof 

 of the more remote period at which a migration to the 

 shores of Australia was possible. A cold current, which 

 sweeps up the western shores of South America, may 



