340 MM 



"-! 



two principal hypotheses that may be imagined to explain 

 this dimorphism. 



The following is a translation of the article referred to as 



giving the first intimation of the author's observations (Ball. 



Soc. Geol. France, ser. 3, tome vui. p. 300) : 



" M. Munier-Chalmas announced to the Society that his 

 researches upon Nummulites laevigata, planulata, variolaria, 

 irregularis, and upon Assilina granulata and spira, have led 

 him to conclude that these species are dimorphic. It is pro- 

 bable that this fact will prove to be general. 



" When we find in the same deposit Nummulites of very 

 different dimensions which have externally the same specific 

 characters, we very soon remark, on breaking them, that the 

 small individuals have a very large central chamber, while 

 that of the individuals of large size is comparatively very 

 small ; and as there are no intermediates between these two 

 forms, they have been made into distinct species. But, on 

 the other hand, as we never find the young of the small- 

 chambered Nummulites above mentioned, M. Munier-Chalmas 

 has been led to regard the latter forms as originating from the 



individuals with large chambers, which are associated with 

 them in most cases. From this he considers it results : — 



u 1. That the individuals with large chambers continue to 

 increase externally at the same time that they absorb their 

 large central chamber, and that in its place they prolong their 

 spiral inwardly, probably in consequence of a spiral inrohnent 

 preexisting in the embryo. 



H 2. That the individuals which become arrested in their 

 development retain their large chamber without modification : 

 thus, for each of these species, they constitute a peculiar stage 

 corresponding to an arrest of development. 



" In the list, in order to avoid confusion between these two 

 stages, one might prefix to the specific name of the individuals 

 with large chambers the designation prw, merely indicating a 

 first stage of development. To cite only one example, we 

 should thus have Nummulites laevigata for the large individuals 

 with small central chambers, and N. prcelcevigata for the N. 

 Lamarckiiy or first evolutive phase of N. laevigata. If this 

 theory of dimorphism among the Nummulites is verified, it 

 will be necessary to diminish considerably the number ot 

 species." 



To the above statements M. P. De la Harpe replied at 

 considerable length in a paper read before the Geological 



