

Dimorphism of the Foraminifera. 337 



we have pursued our researches upon the structure and orga- 

 nization of the principal genera of Miliolidse : — Biloculina 1 

 Dillina, Fabulina, Lasazina, Trilocidina, Trillina, Quinque- 

 loculina, JPentellina, and Heterillina. 



It appears from our recent observations that the dimorphism 

 first discovered in the Nummulites occurs also in all the 

 species of Miliolidse that we have studied ; and that it is there- 

 fore manifested in both the great divisions of the Forami- 

 nifera, Perforata and Imperforata. 



The better to display this character it is necessary to notice 

 the general plan of structure of the three principal genera of 

 Miliolidae. 



The plasmostracum* of the Biloculince, Trilocidince, and 

 Quinqueloctdinre may be regarded, from a schematic point of 

 view, as formed by a tube coiled round a sphere (central cham- 

 ber) and presenting, at each half revolution, a constriction 

 which bounds a new chamber larger than the preceding one. 

 The coiling is effected sometimes in a single direction ; 

 sometimes, on the contrary, at each half revolution the new 

 chamber departs more or less from the preceding one, and the , 

 coiling then follows certain definite directions which pass 

 through the plane of symmetry of the serial chambers. 



In the Biloculince the coiling, taking place in a single 

 direction, remains in the same plane of symmetry, which is 

 consequently common to the two rows of serial and op- 

 posite chambers, which surround a central spheroidal initial 

 chamber. 



The Triloculince are coiled in three directions, which give 

 origin to three planes of symmetry making an angle of 120' 

 with each other. From this arrangement it results that the 

 central chamber is surrounded by three rows of serial chambers. 



Lastly, in the Quinqueloculince 7 which present around the 

 central chamber five rows of the serial chambers, the coiling 

 follows five directions, which define the same number of 

 planes of symmetry, making with each other an angle of 72°. 



The dimorphism of the Foraminifera is characterized by a 

 difference in the size and arrangement of the first chambers. 

 If we make transverse sections of any of the species that we 

 have investigated, we very soon ascertain that the individuals 

 composing them present two types of organization : the smaller 

 ones and those of medium size have always a relatively very 

 large central chamber (form A), while in the larger specimens 

 this central chamber is only visible with a high magnifying 

 power (form B). In the same species there is no external 



* Test of the Foraminifera. 





