178 
Other circumstances may, therefore, contribute to the supposed 
restriction in distribution, which still await direct observations, 
empty speculations being here of little value. 
In my table Plehn’s Stylochus pilidium from Valparaiso is not 
recorded, and this I have purposely omitted. Even if the deter¬ 
mination is correct, the circumstances connected with the find (on 
a ship Corning from Italy) forbid its being taken into account. 
Meixner also .puts a ? in his table (but Brazil is erroneously in- 
dicated for the case). 
It is undoubtedly remarkable that only two Stylochid-genera are 
known in the Atlantic, when all fourteen genera hitherto recog- 
nized are represented in the Indo-Pacific a'rea. This may, however, 
partly be due to the faet that the Polyclad fauna in the tropical 
and Southern Atlantic has remained practically unveiled. But we are 
very probably safe in assuming the former as the main centrum for 
the family. In this area a member of the family, Limnostylochus 
borneensis, has succeeded in invading fresh water and two others 
of the same genus are known to oceur in brackish water. This 
shows a remarkable independence of salinity, the genus being in 
this respect unique among all Polyclads. Otherwise the Stylochids 
seem to be decidedly stenohaline, as they are also very likely 
stenotherme. 
