24 GUIDE TO THE FOSSIL INVERTEBIIATE ANIMALS, 
Gallery X. 
Table-case 
16. 
reproduction of the Foraminifera. Examination of some 
of the pieces of nnmmulitic limestone will .show that 
nummnlites of two sizes are usually associated in the same 
rock. These w’ere formerly supposed to be distinct specie.s, 
and so received distinct names. On splitting the shell of a 
nnmmulite it is found that the spiral series of chambers 
starts Irorn a central spherical chamber, and in these paired 
forms it has been observed that in the larger shell the 
central sphere is of microscoi)ic size, whereas in the smaller 
shell it is readily visible to tbe naked eye (Fig. 5). In 
a b 
Fig. 5. — Two generations of a Nnmmulite. Sections across (a) Nummu- 
litcs laevigatus, showing the small central chamber, and (b) its other 
form known as Nummnlites Lamarcki, showing the large central 
chamber. Both from the Eocene of Stubbington ; enlarged 10 
diameters. (Copied from the original figures by De la Harpe, 1881.) 
consequence of their universal association it was inferred 
that the large circles with small spheres and the small 
circles with large spheres were really two forms of the same 
species ; and it was then found that similar dimorphism, i.e., 
composition of a single species by two forms, occurred in the 
shells of many other genera of Foraminifera. By observation 
of living individuals of one of these {rolystomclla), Schaudinn 
and Lister have proved that the dimorphism results from 
the .alternation of two modes of reproduction : the small- 
sphered form extrudes from the shell the whole of its 
protoplasm, which then separates into spheres, and round 
each of these is deposited a shell wdiich proves to be the 
central chamber of a large-sphered form. In such a form 
the protoplasm ultimately divides into a multitude of minute 
tw'o-tailed particles or spores, wdiich are ejected. When a 
spore from one individu.al meets a spore from another it 
