- 
804 STRATIGRAPHICAL GEOLOGY. [Boox VI. | 
tories. Upwards of 100 species of dicotyledonous angiosperms have 
been obtained, and of these half are found to be related to still living 
American trees. Among them ate species of oak, willow, beech, 
plane, poplar, maple, hickory, fig, tulip-tree, sassafras, sequoia, together 
with American palms (sabal) and cycads. : 
The known Cretaceous fauna is tolerably extensive. Forami- 
nifera now reached an importance as rock-builders which they had — 
never before attained. Their remains are abundant in the white 
chalk of the northern European basin, and some of the hard lime- 
stones of the southern basin are mainly composed of their ageregated 
shells. Some of the more frequent genera are Globigerina, Orbi- 
tolina, Nodosaria, Textilaria, and Rotalia (Fig. 388). Sponges also 
must have swarmed on the floor of the Cretaceous seas, for their 
siliceous spicules are very abundant, and entire individuals are 
not uncommon.’ Characteristic genera 
(Fig. 389) are Ventriculites, Siphonia, 
Scyphia, and Manon. The formation of 
flints has been referred to the operation 
of sponges. Undoubtedly these animals 
secreted an enormous quantity of silica 
from the water of the Cretaceous sea, 
and though the flints are certainly not 
due merely to their action, these amor- 
phous lumps of silica may have been ag- 
gregated by a process of chemical elimi- 
nation round dead sponges (see pp. 469, 
488). Even molluscs and urchins have 
been completely silicified in the chalk. 
25) EE Sent One UE On the whole, corals are not abundant 
Be cae oe ‘ in Cretaceous deposits. Some of the 
a, Siphonia pyriformis (Goldf.) aeKe 
(4); b, Ventriculites decurrens, More characteristic forms are Trochocy- 
var. tenuiplicatus (Smith) (4). athus, Cyathina, T: rochosmilia,\Parasmilia, 
Micrabacia, and Cyclolites. The earliest 
true madrepores appear in Actinacis. The rugose corals so abundant 
among Paleozoic rocks have now almost entirely disappeared, being 
represented only by the little Neocomian Holocystis. Sea-urchins 
are conspicuous among the fossils of the Cretaceous system. A few 
of their genera are also Jurassic, while a not inconsiderable number 
still live in the present ocean. One of the most striking results 
of recent deep-sea dredging is the discovery of so many new genera 
of echinoids either identical with or very nearly resembling those of 
the Cretaceous period, and haying thus an unexpectedly antique 
character.” Some of the most abundant and typical Cretaceous 
genera are Ananchytes, Holaster, Toxaster, Micraster, Hemiaster, Hemi- 
pneustes, Pygurus, Echinobrissus (Nucleolites), Echinoconus (Galerites), 

’ See on sponge spicules papers by Mr. Sollas, Ann. Mag. Nat. Hist. ser, 5, vi., and 
a memoir by Dr. H. G. J. Hinde, Fossil Sponge Spicules, Munich, 1880, 
* A. Agassiz, Report on Echinoidea, Challenger Expedition, vol. iii. p. 25. 
