THE PORIFERA AND PROTOZOA 



325 



(Eoc,, Fig. 98) rectangular ; of Lepidocyclina (Oligo., 

 Fig. 99) more or less hexagonal ; while in Miogypsina 

 (Mio.) there is a more distinct spiral "nucleus." The 

 generic distinctions given above are those most readily 

 made out in thin sections of Limestones, but are not the 

 only ones. 



The Imperforata are regarded as all belonging to one 

 family, Miliolidae. The simplest example is Biloculina 

 (Trias.-Rec., Fig. 96), in which the adult chambers are 

 hemispheroidal, the aperture with a T-shaped central 

 septum, alternately at one end and the other as new 



FIG. 98. ORTHOPHRAGMINA UMBILICATA, DEPRAT, EOCENE, 

 NEW CALEDONIA. 



a, Vertical section. ( X !) b. Portion of a horizontal section. 

 (After Deprat. ) 



(Xio.) 



chambers are added. The early chambers of the micro- 

 spheric form show an arrangement found in the adults 

 of other genera, as Spivoloculina and Miliolina. The 

 family also includes some large, though scarcely giant, 

 forms A Iveolina is a homoeomorph of Fusitlina, but very 

 different in age (Cret.-Rec., maximum in Eocene), and 

 Orbitolites (Eoc.), and Marginopora (Cret. Rec.) resembles 

 the large discoidal genera of the Nummulinidse : the 

 chambers, except in the initial stages, are arranged in 

 cycles with a very regular " engine-turned " pattern, 

 visible externally owing to absence of overlapping. This 



