DISTRIBUTION OF FORAMIKIFERA. 239 



Atlantic, Pacific, and Indian oceans, but, singularly enough, no 

 such reefs have thus far been detected in the deposits in question ; 

 indeed, even the scattered coral fragments are in themselves re- 

 markably scanty, and far from sufficient to give a coralline aspect to 

 the formation ; thirdly, while many of the chalk Mollusca may appear 

 to represent shallow-water forms, there are yet a number, includ- 

 ing some of the most characteristic forms, concerning whose habit 

 nothing positive can be stated, since not only have many of them 

 no living representatives in the faunas of the present seas, but even 

 the families to which they belonged have completely died out. 

 While possibly, then, we may not have as yet arrived at an exact 

 comprehension of the nature of chalk, it must be confessed that 

 the biological facts already indicated point to a very close rela- 

 tionship with the Globigerina ooze. Nor is it at all unlikely that 

 the difference existing in chemical composition is one more appar- 

 ent than real. Thus, the deficiency of silica and, consequently, 

 the surplus of carbonate of lime in chalk can be readily accounted 

 for on the supposition that the free silica originally present in the 

 Cretaceous seas may have sifted itself during the formation of the 

 chalk into those irregular nodules which we now recognise as flints, 

 and likewise into the irregular fissures in the chalk, to form the 

 chalk-veins. Again, it may be assumed that chalk, during the 

 long lapse of ages that has intervened since the period of its forma- 

 tion, may have through various causes undergone considerable 

 alteration in its chemical composition, and sufficient to account for 

 the dissimilarity existing between it and the Globigerina ooze. 



Perhaps the most remarkable feature connected with the history 

 of the Foraminifera is the long period of time through which pri- 

 mary characters have been retained; persistence of type-structure 

 is, indeed, immeasurably better marked in this group of organisms 

 than in any other. It is true that the views of naturalists are very 

 much at variance as to the proper limitations to be assigned to the 

 beings composing this most difficult group of the Invertebrata, and 

 that some of the most eminent authorities are disposed to unite the 

 greater number of both recent and fossil forms into a comparatively 

 limited number of diverging or central types ; but, even from the 

 more conservative standpoint, enough is patent to indicate a most 

 extraordinary specific longevity. This is most clearly brought out 

 by the data furnished in Mr. Brady's report on the "Challenger" 



