REPORT ON THE HEXACTINELLIDA. 



461 



From this summary it will be seen that at depths from 95 to 200 fathoms the 

 probability of a dredging including a Hexactinellid is somewhat large {17 to 80 per cent.), 

 and similarly in depths from 301 to 700 fathoms. Whether the negative results of the 

 Challenger dredgings or trawlings for the depth 201 to 300 and 701 to 900 are really so 

 striking as they at first sight appear, I shall leave as a moot point ; the very small 

 absolute number of dredgings or trawlings in these depths must obviously be taken into 

 account. 



Within the zones from 901 to 2500 fathoms the abundance of Hexactinellida 

 appears to remain approximately constant at from 40 to 50 per cent. Between 

 2501 and 3000 a marked decrease in the number of species (to 12*4 per cent.) 

 takes place, whUe at depths beyond 3000 fathoms no Hexactinellida at all were 

 dredged. 



If a tabular survey, like that given above for the Hexactinellida as a whole, be taken 

 of the several subdivisions, the following statistic shows the relative distribution in the 

 first place of Lyssacina and Dictyonina. 



From the following table also it will be seen that at the slight depth of 95 fathoms 

 only Lyssacina were dredged, and that Dictyonina were unrepresented, but that at 

 depths from 101 to 1000 fathoms Dictyonina occur in equal or slightly greater 

 abundance. 



On the other hand the Dictyonina are far excelled by the Lyssacina in all depths 

 beyond 1000 fathoms. 



