ON THE LIFE-HISTORY OF THE FORAMINIFERA. 113 



as the result of the investigations of four fossil species of 

 Nummulites and two of the allied genus Assilina, his conclusion 

 that these species were dimorphic, and that probably the 

 phenomenon of dimorphism was general. This conclusion was 

 based on the fact that Nummulites occurring in the same 

 strata, and having identical external characters, fall into two 

 groups. These differ from one another, first, in the size of the 

 whole disc; second in the size of the central chamber. On 

 breaking them open it is found that the small ones have a large 

 central chamber, and the large ones a small central chamber. 

 Intermediate forms, it was stated, do not occur. Although the 

 superficial markings are alike in the two forms, they had 

 hitherto been reckoned as distinct species. 



Relying on the fact that only large examples of the forms 

 with small central chambers had been found, Munier-Chalmas 

 concluded that each species consisted of two sets of individuals; 

 first, those with a large central chamber, which attained a small 

 size, and whose development was arrested at this stage ; second, 

 those in which the size was increased by the addition of 

 chambers at the end of the spiral, while the large central 

 chamber was replaced by a prolongation of the spire of 

 chambers to the centre of the shell. 



D'Archiac and Haime (1) had described the two kinds of 

 arrangement of the central chambers in this family, and pointed 

 out the corresponding difference in the size of the whole shell. 

 These characters are referred to (pp. 63, 64, and 77) as indica- 

 tive of different species, though this view is not consistently 

 followed throughout. 



The view stated by Munier-Ch ALMAS, that the two types 

 represent different forms of the same species, had been 

 previously expressed, though less definitely, by Parker and 

 Jones (27 a). These authors, on commenting on d'Archiac 

 and Haime's ' Monographie,' refuse to recognize the mode of 

 arrangement of the central chambers as a character on which to 

 separate natural groups, and state their opinion that the forms 

 of small size, with a large central chamber are "free growing 

 individuals soon arriving at theii" limit of growth." 



