THE FORAMINIFERA 133 



only by the two large chambers which succeed the megalosphere, 

 and the heterostegine stage is considerably shortened. In fact, 

 we find the same tendency in the megalospheric form to abridge 

 or omit the stages repeated by the microspheric form as we have 

 seen in other cases. 



The genus Fusulina is represented by a series of forms which 

 abound in the Carboniferous and Permian rocks in Russia, North 

 America, Sumatra, and elsewhere. By their perforate walls, their 

 bilateral sym- 

 metry about a 

 median plane, 

 and the charac- 

 ter of the aper- 

 ture, which is a 



c 



FlO. SO. 



Forms of the genus Fusvlina from the Carboniferous formation of Russia, 

 showing varying degrees of elongation in the direction of the spiral axis, com- 

 parable with those of the Miliolid Alveolina. A, F. aequalis (d'Eichwald) ; B, F. 

 sphaeroidta (Ehrbg.) ; C, F. princeps (Ehrbg.) ; D, F. cylindrica (Fischer). From 

 Brady, Ann. and Mag. of Nat. Hist. 4, xviii. p. 414. 



slit left between the margin of the septum and the surface of 

 the preceding convolution, they appear to belong to the Nurn- 

 muline stock. Like the species of Alveolina they present varying 

 degrees of elongation in the direction of the spiral axis from the 

 biconvex discs of F. aequalis (Fig. 59, A) to the fusiform tests of 

 F. cyclindrica (D). 



The megalospheric and microspheric forms of Fusulina have 

 been recognised by Schlumberger (58). 



We may now take a brief survey of some of the main pheno- 

 mena which have presented themselves in the several groups. 



Among the species of Foraminifera we meet with modifications 

 of form of three kinds. There is the modification which occurs 

 during the growth of an individual, producing the "multiform" 

 condition of test. There is the difference among individuals 

 dependent on their mode of origin, whether from a megalosphere 

 or a microsphere, which finds its expression in dimorphism. Finally, 

 there is the variation commonly presented to a greater or less 

 extent by animals and plants, the departure of the individual in 

 different degrees from the type form of the species. 



We may consider these three kinds of modification in the 

 reverse order. 



The Variation of the Foraminifera. It has long been recognised 

 by systematists that in many cases the limits of the characters of 

 the species of Foraminifera do not admit of being drawn with any 



