492 foramintfera : — alveolina; orbitolites. 



subdivision of the principal Chambers into Chamberlets. The 

 Chamberlets of each row are connected with each other, as in the 

 preceding type, by a continuous gallery ; and they communicate 

 with those of the next row by a series of multiple Pores in the 



Fig. 249. 



Alveolina Quoii :—a, a, septal plane, showing multiple pores. 



principal septa, such as constitute the external orifices of the last- 

 formed series, seen on its Septal Plane at a, a. 



376. The highest development of that Cyclical plan of growth 

 which we have seen to be sometimes taken-on by Orbiculina, is 

 found in Orbitolites; a type which, long known as a very abundant 

 fossil in the early Tertiaries of the Paris basin, has lately proved 

 to be scarcely less abundant in certain parts of the existing Ocean, 

 whilst it attained a gigantic development in that very early 

 Geological period known as the Silurian. The largest recent 

 specimens of it, sometimes attaining the size of a sixpence, 

 have hitherto been obtained only from the coast of New Holland 

 and various parts of the Polynesian Archipelago ; but disks of 

 comparatively minute size (from the diameter of an ordinary pin's 

 head to that of a small pea) and of simpler organization, are to be 

 found in almost all Foraminiferous sands and dredgings from the 

 shores of the warmer regions of the globe, being especially abundant 

 in those of some of the Philippine Islands, of the Bed Sea, of the 

 Mediterranean, and especially of the iEgean. When such Disks 

 are subjected to Microscopic examination, they are found (if 

 uninjured by abrasion) to present the structure represented in 

 Fig. 250 ; where we see on the surface (by incident light) a 

 number of rounded elevations, arranged in concentric zones around 

 a sort of nucleus (which has been laid-open in the figure to show 

 its internal structure) ; whilst at the margin we observe a row of 

 rounded projections, with a single aperture or pore in each of the 

 intervening depressions. In very thin Disks, the structure may 

 often be brought into view by mounting them in Canada Balsam 

 and transmitting light through them ; but in those which are too 

 opaque to be thus seen-through, it is sufficient to rub-down one of 

 the surfaces upon a stone, and then to mount the specimen in 

 Balsam. Each of the superficial elevations will then be found to 

 be the roof or cover of an ovate cavity or Chamberlet, which 

 communicates by means of a lateral passage with the chamberlet 



