76 MANUAL OF ZOOLOGY. 



Most of the recent Foraminifera are very minute, often 

 wholly microscopic in their dimensions ; but some of the ex- 

 tinct forms attained the size of as much as three inches in 

 circumference (e.g., the Nummulites of the Eocene, fig. 16), 

 and the spheres of the Cretaceous Parkeria may have a cir- 

 cumference more than twice as great as this. 



DISTRIBUTION OF FORAMINIFERA IN TIME. Remains of 

 Foraminifera have been found in Palaeozoic, Mesozoic, and 

 Kainozoic formations. In the oldest stratified rocks with which 

 we are acquainted viz., the Laurentian rocks of Canada 

 there occurs a singular body which has been described as the 

 remains' of a gigantic Foraminifer, under the name of Eozoon 

 Canadense. If truly organic, as is doubted by high authorities, 

 it is the oldest fossil as yet discovered. It appears to have 

 grown in reef-like masses resembling the sessile patches of 

 Polytrema* and Calcarina, to both of which, as well as to the 

 extinct Nummulites, it shows a decided affinity. In the Silurian 

 rocks, remains of Foraminifera, some of which are apparently 

 identical with existing genera, have been detected in various 

 places, and it is not impossible that the large Silurian fossils 

 known as Receptaculites and Stromatopora should really be re- 

 ferred to this order. Little is yet known of the -Foraminifera 

 of the Devonian period ; but the remains of these organisms 

 are found abundantly in the Carboniferous, and less plentifully 

 in the Permian deposits. Whole beds of the Carboniferous 

 Limestone in Russia, Armenia, N. America, &c., are made up 

 of the shells of Fusulina ; and in Britain Mr Brady has shown 

 that the same formation is occasionally largely composed of 

 the arenaceous spheres of Saccammina, a genus which is espe- 

 cially interesting, as Sars has found vast numbers of a living 

 form at considerable depths in the North Sea, and as it is 

 known to occur in rocks as old as the Lower Silurian. In the 

 Secondary rocks Foraminifera occur in great abundance, the 

 widely spread formation known as the Chalk being crowded 

 with these organisms. Chalk itself, in fact, is very largely 

 composed of the cases of Foraminifera, some of which are iden- 

 tical with species now existing. 



In the Tertiary rocks the Foraminifera attain their maximum 

 of development, both as regards the size and the number of 

 the forms which characterise them. The period of the Middle 



* Polytrema is a little branched coral-like Foraminifer, composed of a 

 calcareous test forming a number of irregular chambers, which communicate 

 with one another by wide orifices, and are filled with colourless sarcode. 

 The walls of the chambers are also penetrated by an extensive system of 

 capillary canals. 



