to strata of this age. The _Siphonioe_ (fig. 188) consist
of a pear-shaped, sometimes lobed head, supported by a longer
or shorter stern, which breaks up at its base into a number of
root-like processes of attachment. The water gained access to the
interior of the Sponge by a number of minute openings covering
the surface, and ultimately escaped by a single, large,
chimney-shaped aperture at the summit. In some respects these
sponges present a singular resemblance to the beautiful "Vitreous
Sponges" (_Holtenia_ or _Pheronema_) of the deep Atlantic; and,
like these, they were probably denizens of a deep sea, The
_Ventriculites_ of the Chalk (fig. 189) is, however, a genus
still more closely allied to the wonderful flinty Sponges, which
have been shown, by the researches of the Porcupine, Lightning,
and Challenger expeditions, to live half buried in the Calcareous
ooze of the abysses of our great oceans. Many forms of this genus
are known, having "usually the form of graceful vases, tubes, or
funnels, variously ridged or grooved, or otherwise ornamented
on the surface, frequently expanded above into a cup-like lip,
and continued below into a bundle of fibrous roots. The minute
structure of these bodies shows an extremely delicate tracery
of fine tubes, sometimes empty, sometimes filled with loose
calcareous matter dyed with peroxide of iron."--(Sir Wyville
Thomson.) Many of the Chalk sponges, originally calcareous, have
been converted into flint subsequently; but the Ventriculites
are really composed of this substance, and are therefore genuine
"Siliceous Sponges," like the existing Venus's Flower-Basket
(_Euplectella_). Like the latter, the skeleton was doubtless
originally composed, in the young state, of disconnected six-rayed
spicules, which ultimately become fixed together to constitute
a continuous frame-work. The sea-water, as in the recent forms,
must have been admitted to the interior of the Sponge by numerous
apertures on its exterior, subsequently escaping by a single
large opening at its summit.

[Illustration: Fig. 187--_Kotalia Boueana_.]

[Illustration: Fig. 188.--_Siphonia ficus. Upper Greensand. Europe.]

[Illustration: Fig. 189.--_Ventriculites simplex_. White Chalk.
Britain.]

Amongst the _Coelenterates_, the "Hydroid Zoophytes" are represented
by a species of the encrusting genus _Hydractinia_, the horny
polypary of which is so commonly found at the present day adhering
to the exterior of shell