For the present, and until we are better acquainted with the characters of the Radiolarian fauna at different depths, we may distinguish provisionally the following _five bathymetrical zones_:--(1) The _pelagic_ zone, extending from the surface to a depth of about 25 fathoms; (2) the _pellucid_ zone, extending from 25 to 150 fathoms, or as far as the influence of the sunlight makes itself felt; (3) the _obscure_ zone, extending from 150 to 2000 fathoms, or from the depth at which sunlight disappears to that at which the influence of the water containing carbonic acid begins and the calcareous organisms vanish; (4) the _siliceous_ zone, extending from 2000 or 2500 to about 3000 fathoms, in which only siliceous not calcareous Rhizopoda are found, and in which the peculiar conditions of the lowest regions have not yet appeared; (5) the _abyssal_ zone, in which the accumulation of the oceanic deposits, and the influence of the bottom currents, create new conditions of existence. So far as our isolated and incomplete observations of the zonarial Radiolarian fauna extend, it appears that the subclass Porulosa (SPUMELLARIA and ACANTHARIA) predominates in the two upper zones, and as the depth increases is gradually replaced by the subclass Osculosa (NASSELLARIA and PHÆODARIA), so that the latter predominates in the two lowest zones. The obscure zone which lies in the middle is probably the poorest in species. In general, the morphological characters of the zonarial fauna appear to change gradually upwards into the delicate form of the pelagic and downwards into the robust constitution of the abyssal; so also the average size of the individuals (within the limits of the same family) appears to increase upwards and decrease downwards.