338 MM. Munier-Chalmas and Schlumberger on the 



character, except that derived from the size, to lead us to 

 suspect this fact. There exist further, between these two 

 forms, other differences, which we will now indicate. 



Form A. — The numerous sections that we have made of 

 individuals belonging to this form have always shown us that 

 they had a large central chamber, of a spheroidal form, with 

 thin walls, the diameter of which varies from 200 to 400 /-t. 

 The first chambers surrounding it, in the great majority of the 

 species, have a direction and arrangement like those of the 

 last. 



A. Fig. 1 (magn. 12). B. Fig. 2 (magn. 28). 



Biloculina depressa. 



Biloculina depressa, d'Orb. (fig. 1), which lives in the 

 Atlantic Ocean, may serve as an example. Its central cham- 

 ber is surrounded by chambers which, from their first appear- 

 ance, indicate the most simple Biloculinar type, that is to 

 say, coiling in a single direction. The first of the serial 

 chambers, which is often narrower than the following ones, 

 is in communication with the central chamber through a small 

 circular aperture. 



Form B. — Although the individuals belonging to this 

 form are always the largest, transverse sections passing strictly 

 through the centre are very difficult to obtain. The initial 

 chamber, which is likewise spheroidal, is of extreme smallness 

 in comparison with that of the preceding form, its average 

 diameter hardly exceeding 18-25 ft. In all the species that 

 we have studied the first chambers which appear group them- 

 selves by five around the central chamber, in accordance with 

 five directions, which recall the mode of development of the 

 Quinqueloctdince and Pentellinca ) but soon, either suddenly or 

 by gradual transition, the coiling changes, and the new 

 chambers are arranged, exactly, according to the species, like 

 those of the corresponding form A. The section * of B. de- 



* The figure represent* only the central part of the section ; the last 

 two chambers are wanting-. 



