u8 



PROTOZOA. 



_Fig. 27. — Section of hardened Glo- 

 bigerina ooze, enlarged about twelve 

 times. Recent. (Original.) 



pointed out hereafter, of satisfactory explanation. There are, how- 

 ever, other differences between the two, especially as regards the 

 characters of the Molluscan fauna of the latter, which would indi- 

 cate that the Globigerina ooze and the 

 White Chalk were not formed under 

 conditions precisely alike. 



As regards the distribution of the 

 Foraminifera in time, representatives of 

 the group are found in almost all, or 

 in all, rock-groups in which limestones 

 are developed from the Ordovician 

 period onward. Leaving the much 

 disputed Eozoon on one side, no re- 

 mains of Foraminifera have hitherto 

 been recognised as occurring in any 

 deposit older than the Ordovician. As 

 regards the older rocks, the remains of 

 Foraminifera are usually found most 

 abundantly in limestones, or in calca- 

 reous shales, and particularly in the 

 shaly partings which separate calcare- 

 ous bands. In many cases, they have 

 contributed notably to the composi- 

 tion of the solid crust of the earth, and have often built up massive 

 and widely extended limestones. In other cases, where they are 

 present in smaller quantity, they may be commonly detected in thin 

 sections of limestone. In some instances, the tests of Foraminifera 

 have been infiltrated, prior to fossilisation, with glauconite (silicate 

 of iron and potash), and the actual shell has subsequently been dis- 

 solved away. In some of the warmer seas of the present day, 

 green sands are found to be in course of deposition, the grains of 

 which are largely composed of internal casts in glauconite of the 

 shells of Foraminifera ; and it is probable that the green grains 

 of various deposits, and especially of the Cretaceous system, are 

 in part of a similar nature. 



If we omit the group of the Receptaaditidce, regarded by Hinde 

 as referable to the Porifera, the Ordovician, Silurian, and Devonian 

 limestones exhibit in general a remarkable, and, in fact, an unac- 

 countable, absence of the remains of Foraminifera ; since the variety 

 of the types of these organisms represented in the limestones of the 

 Carboniferous period would justify the conclusion that the order is 

 one of very high antiquity. The remains of Foraminifera are not, 

 however, absolutely wanting in these ancient deposits, since the 

 recent genus Saccammina has been found to occur in rocks of 

 Ordovician age, while the Devonian limestones occasionally con- 





