504 



FORAMINIFERA '. POLYTREMA ; ROTALIA. 



a Rotaline spire, but soon undergoes a change from the Spiral to 

 the Cyclical mode of increase ; the rings being divided into 

 Chamberlets, which are again partially subdivided, and the hollow 

 of the cone being more or less completely filled up by a secondary 

 chamber-growth.* Another very curious modification of the 

 Kotaline type is presented by Polytrema, which so much resem- 

 bles a Zoophyte as to have been taken for a minute Millepore ; 

 but which is made up of an aggregation of Globigerine chambers 

 communicating with each other like those of Tinoporus, and differs 

 from that genus in nothing else than its erect and usually branching 

 manner of growth, and the freer communications between its 

 chambers. 



387. In Rotalia, properly so called, we find a marked advance 



towards the 

 Fig. 255. highest type of 



Foraminiferal 

 structure ; the 

 partitions that 

 divide the 

 chambers being 

 composed of two 

 laminae, and 

 spacesbeingleft 

 between them 

 which give pas- 

 sage to a system 

 of canals whose 

 general distri- 

 bution is shown 

 in. Fig. 255. 

 The proper walls 

 of the chambers, 

 moreover, are 

 thickened by an 

 extraneous de- 

 posit, or ' inter- 

 mediate skele-| 

 Section of Rotalia Schroelteriana near its base and £ on ' which/ 

 showing, a a, the radiating interseptal „ nm ' t .' p „ fnr J 

 i v> 1 - f, 1 w.r,.K™-.o • /. c QnoTTo^oo boxneiimebiorma 



radiating out-' 

 growths ; but 



this peculiarity of conformation is carried much further in the 

 Genus which has been designated Calcanna from its resemblance 



* This curious type was first described by Prof. Williamson in his 

 " Recent Foraminifera of Great Britain," published by the Ray Society, 

 1858; and to it are probably to be referred the fossils (1861, p. 309) 

 described by Mr. Garter ("Ann. of Nat. Hist.," 3rd Ser., Vol. viii.) under 

 the names Conulites and Orbitalina. 



parallel to it 



canals ; b, their internal bifurcations ; c, a transverse 



branch ; d, tubular wall of the chambers. 



