186 GEOLOGY. 



yet in the main ancestral, and are now extinct. The sharks and rays 

 were chiefly of the modern types, though not of living species. 



The marine invertebrates. — The most notable departure from the 

 precedents of the preceding ages is the prominent place which the 

 rhizopods or foraminifers took in the record. They made large con- 

 tributions to the distinguishing formation of the period, the chalk, 

 and they were concerned in the formation of the greensand, scarcely 

 less characteristic of the period than the chalk. While these minute 

 organisms live on shallow bottoms, on fixed alga?, and in abysmal 

 water, they are chiefly denizens of the surface waters of the open sea. 

 It is not essential to them that the sea be deep, but in shallow seas the 

 relatively large amount of terrigenous material deposited, the mechani- 

 cal action of clastic material, and the prevalence of higher forms of life 

 that prey upon them, render the accumulation of their shells in dis- 

 tinctive deposits rare, while in the abysmal waters, where these hostile 

 agencies are essentially absent, foraminiferal oozes are characteristic 

 formations. On this account, it was formerly held that the chalk 

 deposits were of deep-sea origin, and hence implied deep depression 

 of the chalk-areas; and since shallow- water deposits are sometimes 

 intercalated between chalk-beds, profound oscillations of level were 

 freely deduced. But the presence in the chalk of the fossils of shallow- 

 water life, joined to other considerations, has forced the essential aban- 

 donment of this view. The relative prominence of the foraminifers 

 becomes all the more curious on this account. The breadth of the 

 epicontinental seas, the lowness of much of the land, and its ample 

 vegetal mantle, sufficiently explain the restriction of clastic com- 

 petition and the associated destructive action; but they leave the 

 relative scantiness of the usual invertebrate life of clear and shallow 

 seas unexplained. Two suggestions of uncertain value may be offered: 

 (1) the water, though not deep in the abysmal sense, may have been 

 somewhat too deep over the chalk-areas to furnish congenial condi- 

 tions for most of the invertebrates, and (2) the limitation of the fresh- 

 water supplies of food usually borne out by the rivers may have affected 

 adversely the food-supply upon which the shallow-water invertebrates 

 depend. 



Sea-urchins were quite abundant, and lent one of its characteristic 

 aspects to the fauna (Fig. 417, q-u), while corals and crinoids, so long 

 associated with clear seas, were not abundant, facts which lend some 



