STRUCTUEE AND EEPRODUCTION 27 



Ehrenberg the specific name for that form of Sjjiril- 

 lina^ namely, >S'. rivipara. 



In his ' Introduction ' Dr. Carpenter in 1862 

 mentions the fact that Professor Kitchen Parker had 

 noticed young specimens having an investing shell 

 occupying the peripheral chambers of Orhitolites; 

 and soon after, another independent observer, Semper, 

 noticed the same thing. 



This leads us to another side of the same 

 question, namely, the phenomenon of dimorphism ; 

 for all the specimens found in the peripheral chambers 

 of Orhitolites have invariably a comparatively large 

 primordial cell or chamber. 



The dimorphism referred to consists in the same 

 species exhibiting two distinct forms of shell. It 

 was shown by Munier Chalmas in 1880 that Num- 

 mulites occurring in the same strata, and having 

 the same external characters, fall into two groups ; 

 one possessing a large central chamber and a small 

 disc, the other a small central chamber and a large 

 disc. These are respectively termed form A and 

 form'B, or the megalospheric and microspheric stage 

 of the species. This phenomenon is best seen in a 

 half-section of the shell taken through the middle of 

 the test, or in a thin median section (fig. 17). 



Further observations on these two types of shell 

 form were made by De la Harpe on Nummulites, and in 

 1883 and 1885 by Schlumberger in collaboration with 

 Munier Chalmas on dimorphism in the Miliolid.e. 

 These authors have shown that in this group the inter- 

 nal arrangement of the chambers is different for the 



