REPORT ON THE HEXACTINELLIDA. 



463 



gradually from 21 to 3 per cent. It is therefore evident that in general the Dictyonina 

 live at much less depths than do the Lysaacina, which are distributed with much greater 

 uniformity over the entire bathymetrical range of the HexactineUida. 



If the consideration of these relations be extended to the several families (subdivisions 

 in the case of the Dictyonina), very remarkable differences obtain in regard to the four 

 families of Lyssacina. In spite of the absolute smallness of the numbers, the diflferences 

 seem striking enough to merit notice here. 



In regard to the Euplectellidse, including Euplectella aspergillum, which lives at 

 a depth of 95 fathoms, the following table shows the distribution in regions of 500 

 fathoms : — 



Euplectellids are thus widely represented from a depth of 95 fathoms {Euplectella 

 aspergillum) down to 3000 fathoms, and so far as an estimate with intervals of 500 

 fathoms is a guide they appear to be distributed with tolerable uniformity at all depths. 

 That no Euplectellids were found on the Challenger Expedition between 100 and 500 

 fathoms is probably accidental. It is an interesting fact that the only EuplecteDid 

 occurring at a depth of less than 100 fathoms, viz., Euplectella aspergillum, belongs to 

 those forms which, like the Dictyonina, exhibit a connected latticed framework. 



Like the Euplectellidse, the Asconematidse are represented down to the lowest zone. 

 The largest number of species occurs at depths between 1501 and 2000 fathoms, and fi'om 

 that region the number decreases both upwards and downwards, as is showTi in the 

 following scale : — 



