106 Testaceous BMzopods from the [""jZrikfSXf 
abundant to constitute a considerable percentage. Diatomaceae 
and Dictyocbidae, moreover, whicb bave indubitably lived at tbe 
surface and sunk down to tbe bottom only after death, occur 
unmetamorpbosed in tbese deposits; and in no instance that bas 
fallen under my observation, bas there been any evidence tending 
to denote even partial disintegration of their shells and skeletons, 
either by gradual mechanical, or chemical, action taking place at 
the bottom of the sea. That no such disintegration does take place 
in the case of the Polycystina, even during the lapse of long 
geological periods, the fossil earths already referred to bear witness. 
The inference from tbese facts is obvious and highly important in 
its bearing upon the view which bas been recently propounded as 
to the present being, in reality, a continuation of the cretaceous 
epoch. For it is clear that the absence of Polycystina from the 
chalk must either be satisfactorily accounted for (as they must 
originally have been present in it whilst it was being deposited, if 
the conditions of the sea-bed were the same then as now) or, to this 
extent at least, the present deposits cannot be regarded as analogous 
to those with which it is customary to compare them. 
In the case of the Acanthometrae there is, however, an im- 
portant difference. The action on these organisms of strong- 
boiling nitric or nitro-hydrochloric acids shows them to be imper- 
fectly siliceous. But no amount of subjection to boiling acids 
produces the slightest effect upon the shells of the Polycystina, 
Dictyochidse, or the minute shells of Cadium and Protocystis about 
to be described in this paper. It is, therefore, both possible and 
probable that the sub-siliceous nature of the acanthostypes of tbe 
Acanthometrae causes them to be so rapidly acted on, after death, 
by the alkaline constituents of the sea-water, that they are disin- 
tegrated before they reach the bottom. At all events there seems 
to me to be no other mode of accounting for their entire absence 
from the deposits, notwithstanding that they occur abundantly at 
the surface of the ocean, although not always in the same latitudes.* 
Accordingly, whilst the absence of the Acanthometrae from the 
post-tertiary fossil earths is only in accordance with their absence 
from the deposits now in the course of formation at the bottom of 
certain seas, it does not — as in the case of the Polycystina still 
found living at the surface of the ocean, and, I beheve, only found 
* I allude here to the transporting agency of the Gulf Stream, by means of 
which the frustules of many free-floating surface Diatoms, such as Asterolampra 
and Bemidiscus, and the like (which are strictly inhabitants of tropical and sub- 
tropical seas), are gradually transferred to the North Atlantic, probably die under 
the change of climate thus entailed on them, and then subside to the bottom as 
component parts of the deposits there forming. It is true the examples are rare, 
but they nevertheless suffice to show that the Acanthometrae, which are not con- 
fined to the smface- waters of tropical and sub-tropical seas, but occur in our own 
latitudes, may be in like manner transported. 
