6o THE FORAMINIFERA 



They are often so abundant that, as in the rock of which some of 

 the Egyptian pyramids are built, their coin-like shells, whole or in 

 fragments, constitute the main part of the deposit (Fig. 5). The 

 shells are, in reality, not flat but biconvex discs, and the chambers, 

 arranged in a spiral, are so disposed that the greater part of the 

 cavity of each lies in the median plane, while the shell on either 

 side of this plane is comparatively solid. They thus readily 

 break, as the result of weathering or by artificial means, into 

 plano-convex halves, which display a section of all the chambers 

 from the centre to the periphery on their broken faces. 



It has long been recognised that while the great majority of 

 the specimens of nummulites occurring in a deposit attain a 

 certain, moderate size, a few are found scattered through it, whose 

 diameter far exceeds that of the others. On examining median 

 sections of the smaller specimens it is usually found that the 

 spiral series of chambers starts from a large and nearly spherical 

 chamber readily visible to the naked eye, and occupying the 

 centre of the shell, while in the larger specimens the spiral series 

 is continued to the centre, where, in carefully prepared sections, 

 it may be seen to take its origin in a spherical chamber of micro- 

 scopic size (Fig. 6). 



Although the two forms were thus found to be associated in 

 the same beds, and to agree with one another closely except in 

 the size to which they grow and the characters of the central 

 chambers, they were given separate specific names, and attention 

 was called to the puzzling occurrence of these associated pairs of 

 species, a large and a small one, in various deposits. 



Thus the names Nummulites laevigata, Lam., and N. lamarcki, 

 d'Arch., have been given to two associated forms occurring in beds 

 of the Middle Eocene formation. The former attains a diameter 

 of 20 mm., while the latter does not exceed 3 or 4 mm. Small 

 examples of N. laevigata are not to be distinguished by external 

 characters from the associated form, but on splitting them open, 

 the difference in their central chambers is at once apparent 

 (Fig. 6). Sixteen pairs of similarly associated " species " belong- 

 ing to the genera Nummulites and Assilina have been enumerated. 



The possibility that the two associated forms might belong to 

 the same species was, however, entertained by several observers, 

 and the acceptance of this view was accelerated by Munier- 

 Chalmas (26), who (in 1880) definitely formulated the conclusion 

 that the species of nummulites are dimorphic, each appearing 

 under two forms, a large one and a small one. He also expressed 

 the opinion that the phenomenon of dimorphism would be found 

 to be of general occurrence among the Foraminifera. 



As already stated, the large forms with a small central chamber 

 are much less abundant than the others, and it so happened that 



