272 HISTORICAL PALEONTOLOGY. 



divisions 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 

 recognized in the Chalk ; but the three principal genera are 



Fig. 187. Rotalia Boueana. 



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 varie- 

 ties of shape and structure ; but the two most characteristic 

 genera are Siphonia and Ventriculites, both of which are ex- 

 clusively confined to strata of this age. The Siphonia (fig. 

 188) consist of a pear-shaped, sometimes lobed head, supported 

 by a longer or shorter stem, 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 

 flint 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 "usu- 

 ally the form of graceful vases, tubes, or funnels, variously 



