CHAP, v.] GENERAL CONCLUSIONS. 293 



spheres, which probably represent the spheres of a lighter color 

 which we find in all the surface rhizopods, seem to be special- 

 ly characteristic of rhizopods from deep water, being found 

 also in the Radiolarians from the deep tow-nets. The Challen- 

 gerida were never met with on the surface ; they were taken 

 rarely in tow-nets sunk to depths of 300 and 400 fathoms ; and 

 they were most abundant when the tow-nets were sent down 

 on the dredge or trawl rope to nmch greater depths. Their dis- 

 tribution seems to have a wide extension ; they are occasionally 

 found in the bottom deposits, but rarely, probably on account 

 of their small size and the extreme tenuity of their tests, which 

 renders them liable to solution in sea-water. The Challenge- 

 rida are essentially rhizopods with monothalamous siliceous 

 shells ; and their zoological position may be not very far from 

 such forms as Gromia. 



The distribution of the pelagic Foraminifera has already 

 been discussed. They are universally distributed throughout 

 the temperate and warmer seas, diminishing in number and de- 

 creasing in size toward the frigid zones. Certain species are 

 occasionally found in large numbers on the surface, but at a 

 depth of a few fathoms their occurrence is much more certain. 

 We have good reason to believe that the vertical range of the 

 oceanic group does not extend beyond the first few hundred 

 fathoms, and that all the pelagic forms occur occasionally on 

 the surface. Living Foraminifera are very generally distrib- 

 uted on the bottom, but the forms differ from those found on 

 the surface and near it, and are for the most part to be referred 

 to arenaceous or imperforate types. 



Sponges extend to all depths, but perhaps the class attains 

 its maximum development between 500 and 1000 fathoms. All 

 the orders occur in the abyssal zone, except the Calcarea, which 

 seem to be confined to shallow water. At great depths the 

 Hexactinellidee certainly preponderate ; and next to these per- 

 haps the Esperiadse, the Geodidse, and the Lithistidee. The 



