24 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



from the trochospiral coil as seen in Globigerina. The chamber ar- 

 rangements, though sometimes difficult to express geometrically, are 

 useful in distinguishing the species of Globigerinoides. Illustrations 

 of chamber arrangements in Recent species of Globigerinoides are 

 shown in figure 4. 



jjjjwnrt;^-^ 



4d 



Fig. 4, — Chamber arrangements and positions of primary apertures in some 

 species of Globigerinoides. 4a, G. ruber, ventral view. 4b, G. conglobatus, im- 

 mature form, ventral view. 4c, G. conglobatus, mature form, ventral view. 

 4d, G. trilobus trilobus, edge view. 4e, G. trilobus trilobus, ventral view. 



In Globigerinoides conglobatus the chambers are coiled in a fashion 

 similar to that in Pulleniatina obliquiloculata, with the early chambers 

 added in a trochoid spiral and the final chambers added streptospirally, 

 overlapping the umbilicus. In the final stage of development the last 

 chamber completely covers the umbilicus. 



In Globigerinoides ruber there is a pronounced shift in the axis of 

 the chambers as they are added ; in fact, it is somewhat difficult to 

 think of the chambers actually forming a coil. Each chamber is added 

 directly above the aperture of the earlier chamber. The aperture of 

 the new chamber is 90 degrees apart from the aperture of the pre- 



