588 TRANSACTIONS OF SECTION D. 



It has long been recognised that while the great majority of (he 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 large 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 microscopic size. 



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 

 in 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. 



It was especially by the labours of T)e Hantken and De la Harpe that this 

 phenomenon waa brought to light, the latter palaeontologist 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, the small always has one.' 

 Over sixteen pairs of species of Nummulites and the allied genus Assilina, associated 

 in this manner have been enumerated. 



In the year 1880 Munier-Chalmas brought before the Zoological Society of 

 France his conclusion that the kinds thus associated were not in fact distinct 

 species but two forms of the same species — that, in fact, the species of Num- 

 mulites were dimorphic. He also expressed the opinion that the phenomenon of 

 dimorphism would be found to be of general occurrence among the Foraminifera. 



To this view, which further investigations have shown to be entirely correct, 

 Munier-Chalmas added a corollary as to the nature of the relation between these 

 two forms, which was wrong. This, however, need not detain us here." Whether 

 he was set against Munier-Chalmas' views by the error of part of them, or for 

 whatever reason, De la Harpe failed to recognise, before his untimely death which 

 occurred shortly after, the truth which they contained. 



Following up the clue which had been found, Munier-Chalmas and his 

 colleague Schlumberger examined the shells of a large series of forms, especially 

 of the Miliolidce. It was shown, in a fine series of papers, that the phenomenon 

 of dimorphism was present here too, and may find its expression, not only in 

 differences in size of shell and of central chamber, but also in the plan in which 

 the chambers of the two forms are arranged. 



While they differ conspicuously — though, as we shall see, in very varying 

 degrees — in the sizes of the initial chamber, it is by no means the case that in all 

 species, as in those of the genus Isummulites we have considered, the size attained 

 by the completed test presents so marked a difference. It is, in fact, more usual 

 for the individuals of the two forms of a species to attain approximately the same 

 size on the completion of growth, though standing so contrasted in the size of the 

 initial chambers. 



The names megalosphere and microsphere have been given to the large and the 

 small initial chambers, and the two forms are generally known as the megalo- 

 spheric and microspheric respectively. 



The examination of other groups of Foraminifera has abundantly confirmed the 

 view that the phenomenon of dimorphism is widely prevalent among them. 



The Life- history/ of Poly stomellsi crispa. 



Turning now from the consideration of the shells of Foraminifera to the living 

 animals, let us inquire what light has heen gained from them on the problem of 

 the significance of the phenomenon of dimorphism. 



' Usually, because the young of the other type occurs among the smaller 

 specimens. 



* Cp. the article by the author on ' Foraminifera ' in Lankester's Treatise on Zoology, 

 Part I., Fasc. 2, p. 47 ; and ' On the Dimorphism of the English Species of 

 flummulites, &c.' P.B.S., vol. 76 B., p. 298. 



