CHAP. III. ] TENERIFFE TO SOMBRERO. 221 
which are of greatest importance in a geological point of view, 
the Potycysrrma and the AcANTHOMETRINA, the skeleton is much 
more regular and complete. In the former it consists of a del- 
icate external shell of silica, minutely fenestrated, and often 
presenting very remarkable and 
beautiful forms (Fig. 52); in the 
latter it is essentially internal, and 
is formed of a varying number of 
siliceous spicules, radiating from 
a centre round which the sarcode 
is accumulated (Fig. 53). The 
spicules are often most elegantly 
ornamented ; and in an interme- 
diate family, the Haliommatidee, 
they give off a set of anasto- 
mosing branches, which form one 
or several concentric lacey shells 
which invest the sarcode nucleus 
(Fig. 54). 
The observation of the great 
abundance of Radiolarian tests at 
great depths led to the reconsid- 
eration of the deposits from the 
0 F 5 Aa any Fia. 52.—Dictyopodium (sp.n.). From the 
deepest soundings ’ and M : Mur- surface. Two hundred times the natu- 
ray now believes, and in this J ™*% 
entirely agree with him, that shortly after the red clay has 
assumed its most characteristic form, by the total removal of 
the calcareous shells of the foraminifera, at a depth of say 8000 
fathoms, the deposit in many cases begins gradually to alter 
again, by the increasing proportion of the shells of Radiolari- 
ans, until, at such extreme depths as that of the sounding of the 
23d of March, it has once more assumed the character of an al- 
most purely organic formation—the shells of which it is chief- 
ly composed being, however, in this case siliceous, while in the 
I.—15 
