1907.] on the Foraminifera. 491 



with the stadj of the fossil niimmuHtes, from the later Eocene rocks 

 — called nummulites from their likeness to coins. The nummulitic 

 limestoues extend from the Pyrenees to China — they are often thou- 

 sands of feet thick. They form large deposits in Egypt, where the 

 coin-like discs have attracted notice from remote antiquity. 



[Some examples displayed in the library of N . gizehensis from the 

 Fayum and of Algerian nummalitic limestones were here alluded to.] 

 "^ The structure of a nummulite is very like that of Folystomella, 

 but the whorls much more numerous, and the main space of each 

 chamber is in the median plane, hence they readily break in this plane 

 exposing a section of all the chambers. 



It has long been recognised that in a nummulitic deposit a few 

 specimens far exceed the others in size. 



[A lantern slide was here displayed showing a specimen of num- 

 muhtic limestone from the Nile valley.] 



In median sections of the small specimens the spiral series of 

 chambers is seen to start from a large and nearly spherical chamber, 

 readily visible to the naked eye, while in the few large specimens the 

 spiral series is continued to the centre, where in carefully prepared 

 sections it may be found to take its origin in a central chamber of 

 microscopic size. 



[A diagram showing the central regions of the two kinds of shell 

 was here displayed.] 



Although the two forms were found associated and to agree 

 closely except in size and in the characters of the central chambers, they 

 were regarded as belonging to different species, and attention was 

 called to this puzzling " association of species of nummulites in pairs," 

 a large and small one. 



De Hantken and de la Harpe brought this phenomenon to light, 

 the latter formulating his " Law of the Association of Species in Pairs," 

 as follows : " Nummulites appear in couples, each couple is formed of 

 two species of the same zoological group, and of unequal size. The 

 large species is without a central chamber (sic), the small always has 

 one." Sixteen pairs of " species " were enumerated. 



In 1880 Munier-Chalmas propounded his view that the associated 

 kinds were not of distinct species, but two forms of the same species, 

 in fact that the species of nummulites were dimorphic. He expressed 

 the view that this would be found to be general in the Foraminifera. 



[A lantern slide showing the two forms of N. Imvigatus from Selsey 

 Bill was here displayed.] 



Munier-Chalmas and Schlumberger, examining shells of llilioUdcB, 

 found that here too the species are dimorphic, the dimorphism finding 

 its expression in differing arrangements of the chambers, as well as in 

 sizes of central chambers and of the whole shells. 



It is, however, not always or even usually the case that the size 

 attained by the shells of the two forms is different. 



[A lantern slide showing the two forms of N. variolarms from the 



