As regards animal life, the _Protozoans_ of the Cretaceous period are exceedingly numerous, and are represented by _Foraminifera_ and _Sponges_. As we have already seen, the White Chalk itself is a deep-sea deposit, almost entirely composed of the microscopic shells of _Foraminifers_, along with Sponge-spicules, and organic _débris_ of different kinds (see fig. 7). The green grains which are so abundant in several minor subdivisions of the Cretaceous, are also in many instances really casts in glauconite of the chambered shells of these minute organisms. A great many species of _Foraminifera_ have been recognised in the Chalk; but the three principal genera are _Globigerina, Rotalia_ (fig. 187), and _Textularia_--groups which are likewise characteristic of the "ooze" of the Atlantic and Pacific Oceans at great depths. The flints of the Chalk also commonly contain the shells of _Foraminifera_. The Upper Greensand has yielded in considerable numbers the huge _Foraminifera_ described by Dr Carpenter under the name of _Parkeria_, the spherical shells of which are composed of sand-grains agglutinated together, and sometimes attain a diameter of two and a quarter inches. 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. 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