370 
THE VOYAGE OF H.M.S. CHALLENGER. 
and by only three in the northern. More than half the new species in this family 
were obtained to the south of 40° south latitude. 
In the Didemnidse, on the other hand, there is no such disproportion ; the northern 
hemisphere shows eleven entries and the southern fifteen, and the members of the 
family are apparently scattered widely. The genus Leptoclinum is well represented in 
both hemispheres, and has a wider range than any other genus in the collection. It is 
not a markedly southern form, and in this respect it contrasts with Amaroucium, the 
next largest genus, which has nearly all its species confined to the far south. 
The Challenger Diplosomidse are from tropical seas, and the family Coelocormidse is 
only known from the southern hemisphere. 
The Polystyelidae have a wide range. Most of the Challenger species are from far 
south, but one {Synstyela incrustans) also occurs north of the equator. The species of the 
family which were known previously are from the north temperate zone. These, however, 
belong to genera distinct from the southern forms. 
