REPORT 01^ THE TETRACTINELLIDA. 
389 
geographical connection between the Atlantic and Pacific provinces, but in the similar 
climatal conditions of the different areas, which have ensured the survival of species once 
much more widely distributed ; this explanation derives support from the fact that the 
representative species all belong to genera which are markedly peculiar in character, and 
extremely poor in species : the three Lithistid genera cited do not possess so far as we 
know any other species than those mentioned, Placospongia is an aberrant genus with 
but three known species, and Trihrachium and Disyringa are equally aberrant genera 
of Stellettidm, but they are also sufficiently distinct from each other to deprive their 
representative character of any great value. The great cup-shaped Synops of the 
Brazilian province is represented by the cup-shaped Cydonium of Japan ; cup-shaped 
Geodiids occur nowhere else. 
The South Australian province is characterised by the genus Anthastra, which is 
peculiar to it, and which seems to represent the genus Myriastra of the Indo-Pacific 
province; the genus Psammastra with its two species, Psammastra murrayi and 
Psammastra geodides, is also peculiar to this region ; the Theneid — The.nea grayi — is not 
a very distinct form, since it much resembles Thenea muricata of the Arctic province ; 
Chrotella simplex is characteristic and represents Chrotella macellata of the Indo- 
Pacific province ; similarly Cydonium eosaster represents Cydonium globostellifera ; 
Synops nitida is a very distinct and characteristic species. One marked feature which 
appears to distinguish this region from the Indo-Pacific is the rarity of Lithistida : only 
one species — Discodermia discifurca — was obtained by the Challenger, and Dr. von 
Lendenfeld informs me that he did not succeed in obtaining a single specimen from Port 
Jackson. It is possible that this distinction is correlated with difference in climate, Lithis- 
tids preferring warm seas, and thriving best in water constantly over 40° in temperature. 
From the foregoing account it would appear that the distributional areas which serve 
in the case of the Mollusca are, with certain modifications, also applicable to that of the 
Sponges, and furthermore, that the deep-water and shallow- water Sponges are referable 
to the same provinces. 
The existence of these provinces is probably to be explained by the existence of 
ocean currents, and if we had but a complete and exact knowledge of these the mysteries 
of the distribution of marine forms wmuld be, to a great extent, revealed. Where a 
current flows along a coast it will act as a distributing agent, and its action will be aided 
by surface drifts, which on the average will have very much the same direction as its 
own, but when it crosses an ocean almost bare of islands its power as a distributing agent 
IS lost; it is therefore possible that the North Atlantic, though so much narrower than 
the Pacific, acts as a more efficient barrier to the passage of species from one side to the 
other owing to the comparative absence of islands, which in the Pacific are so plentiful ; 
on the other hand where islands lie thickly scattered in the path of a current they cannot 
fail to serve as settling places to crowds of larAml forms, and thus afford stepping stones 
