322 PAL/EONTOLOGY 



a much larger size than the megalospheric ; in others 

 there is no difference in size. Both forms occur together 

 in the same strata, the megalospheric forms being more 

 abundant than the microspheric a natural result of the 

 fact that a number of megalospheric generations intervene 

 between one microspheric and the next. The late Robert 

 Douville noticed that in the last beds in which a given 

 species occurred, megalospheric individuals might be 

 exclusively present an indication of near approach to 

 extinction. 



The Foraminifera have been divided into three orders, 

 based on the structure and composition of the test : 

 (i) Perforata or Hyalina, with test of calcite, transparent 

 (at least in the more primitive forms) and with abundant 

 small perforations in addition to (rarely without) a main 

 aperture in the last formed chamber ; (2) Imperforata or 

 Porcellanea, with shell of aragonite, less translucent and 

 of a brown colour by transmitted light, and without per- 

 forations ; (3) Arenacea, in which the test is formed of 

 foreign bodies, such as sand-grains, sponge-spicules, or 

 the small tests of other Foraminifera, held together by an 

 organic secretion. The first two orders are well defined, 

 but the third is probably a rather mixed assemblage of 

 forms which have been led by special circumstances to 

 adopt this curious way of protecting themselves in place 

 of secreting a shell. 



The five principal families of the Perforata are as 

 follows : 



i. Lagenidae, the simplest forms, with thin trans- 

 parent shell. Genera : Lagena, single-chambered, flask- 

 shaped ; Nodosaria, a straight row of chambers (ortho- 

 cone) ; CrisUllaria, spiral (ophiocone to sphaerocone). 

 These all appear to range from Lower Cambrian to 



