392 
THE VOYAGE OF H.M.S. CHALLENGER. 
It will be interesting to compare the results obtained from the Tetractinellida with 
those from the Monaxonida and Hexactinellida ; the latter are given by Schulze, the 
former I have obtained by analysing the lists of Messrs. Eidley and Dendy, and the 
short one of additional Monaxonids given in Appendix II. of this Eeport. 
Atlantic. 
IndoAntarctic. 
Pacific. 
Hexactinellida, .... 
19-1 
50 
47T 
Tetractinellida, .... 
11-6 
25 
34-71 
Monaxonida, .... 
44-2 
82-1 
76-6 
The poverty of the Atlantic as compared with the other two basins is shown by all 
three groups, but while the Pacific is richer in species than the Indo- Antarctic in the case 
of the Tetractinellida, the reverse is true in the case of the Hexactinellida and the 
Monaxonida. 
In making these observations we must be careful, however, not to read into them 
more than the facts justify ; thus the relative richness of the three areas we have been 
considering is not really the relative richness of these areas at all, but merely of the 
three portions of the track of the Challenger falling within them ; the importance of this 
truistic remark will be seen if we consider how greatly the results we have reached would 
almost certainly have been modified had the Challenger passed right through the axis of 
the Caribbean area, and shaped its course through the West Indian Islands, or if it had 
avoided the East Indies or touched at only one of those islands. To show how favourable 
the conditions presented by the East Indian and associated islands of the Pacific are to 
the rich development of species, I have compared the richness of the eastern half of the 
Pacific with that of the western, the meridian of 170° being chosen as the line of separa- 
tion between the two parts. We then have ; — 
East Pacific, 
79 Stations. 
West Pacific, 
41 Stations. 
Per cent, of 
Stations. 
Per cent, of 
Species. 
Per cent, of 
Stations. 
Per cent, of 
Species. 
Hexactinellida, 
22-79 
65-8 
24-4 
43-9 
Tetractinellida, 
21-52 
45-57 
9-75 
14-63 
Monaxonida, 
24 
86-1 
22 
48-8 
