24 GUIDE TO THE FOSSIL INVEETEBRATE ANIMALS. 



Gallery X. fresh interest owing to a curious problem connected with the 

 reproduction of the Foraminifera. Examination of some 

 of the pieces of nummulitic limestone will show that 

 nummulites of two sizes are usually associated in the same 

 rock. These were formerly supposed to be distinct species, 

 and so received distinct names. On splitting the shell of a 

 nummulite it is found that the spiral series of chambers 

 starts from a central spherical chamber, and in these paired 

 forms it has been observed that in the larger shell the 

 central sphere is of microscopic size, whereas in the smaller 

 shell it is readily visible to the naked eye (Fig. 5). In 



Fig. 5. — Two generations of a Nummulite. Sections across (a) Nummu- 

 lites laevigatas, showing the small central chamber, and [b] its other 

 form known as Nummulites 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 theirj 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 (Polystomella), 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 which proves to be the 

 central chamber of a large-sphered form. In such a form 

 the protoplasm ultimately divides into a multitude of minute 

 two-tailed particles or spores, which are ejected. When a 



