REPORT ON THE HEXACTINELLIDA. 449 



it is seen that 25 species are found north of the equator, as against 47 to the south, 

 while of Dictyoniua 13 l^elong to the north, and 20 to the south hemisphere. 



And since in the north liemisphere there were 126 dredgings or trawlings, as against 

 150 in the south, the percentage proportions for 100 dredgings or trawlings are as 

 follows : — The Lyssacina occurred in the north hemisphere in the proportion of 

 19 "8 per cent., as compared with 3r3 per cent, in the south ; the Dictyonina are 

 represented in the north hemisphere by 10 "3 per cent., as against 13 '3 in the south. 



The absolute number of Lyssacina found in the north tropics was 11, as compared 

 with 18 in the south. And if the number of dredgings or trawlings in the north and 

 south tropics are taken into consideration, the percentage proportions stand as follows — 

 22 per cent, in the north tropical zone, and 367 in the south. 



Of Dictyonina 7 species were found in the north tropical zone, as against 14 species 

 in the south. And this expressed in percentages of dredgings and trawlings means 

 14 per cent, for the north tropics and 28 "6 for the south. 



If we now pass to the consideration of the several families, it will be seen that most 

 of them are represented in all the three zones, though generally in very varying 

 abundance. It is noteworthy that the EuretidiB and Mceandrospongidas are wdiolly 

 absent from the south temperate zone, while the northern region exhibits a similar 

 absence of Tretodictyidse, and only one representative of all the other families of 

 Dictyonina. The tropics on the other hand include representatives of almost all the 

 families, and one at least from among the Asconematidae and Cosciuoporidse. 



WhUe the Euplectellidfe are represented by several forms in all the three zones, the 

 north temperate zone, which contains the genus Ewplectella itself, yielded no Holascidse, 

 which occur both in the tropics and in the south temperate zone. It is a striking fact that 

 the Asconematidae, which occur somewhat abundantly in the south temperate zone and 

 likewise in the north, are represented in the tropics only by a single species. The 

 Rossellidse occur in considerable abundance in the tropics, and in especial abundance in 

 the south temperate zone, but are represented only by one species in the northern region. 



The Hyalonematidse exhibit a tolerably uniform distribution through all the three 

 zones, with a slight preponderance in the tropics. 



The tropics are especially rich in Farreidse and Euretidse, to which have to be added 

 two species of Melittionidse, several Tretodictyidaj, and two species of Mseandrospongidaj, 

 a distribution in striking contrast to that of the two temperate zones, in which these 

 families either do not occur, or are represented only by a single species. 



A more detailed survey of the genera and species shows that the species of Euplectdla 

 occur only in the north temperate zone and in the tropics, being apparently absent in the 

 southern region. Euplectella aspergillum and Ewplectella crassistellata appear to be 

 tropical, while Euplectella nodosa was confined to the north temperate zone, and 

 Euplectella suberea occurred in both these regions. 



^OOL. CHALL. EXP. — PART LIII. — 1887.) Ggg 57 



