The Cretaceous Sponges are
extremely numerous, and occur under a great number of varieties
of shape and structure; but the two most characteristic genera
are _Siphonia_ and _Ventriculites_, both of which are exclusively
confined 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.
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