FORAMINIFERA AND RADIOLARIA. 



73 



after in Orbitolites ( 466). The largest existing examples of this type 

 are far surpassed in size by those which make up a considerable part of a 

 Tertiary Limestone on the Malabar coact of India, whose diameter 

 reaches 7 or 8 lines. 



465. A very curious modification of the same general plan is shown 

 in Alveolina, a genus of which the largest existing forms (Fig. 315) are 

 commonly about one- third of an inch long, while far larger specimens 



FIG. 315. 



FIG. 316. 



Alveolina Quoit : a, a, septal plane, showing multiple pores. 



are found in the Tertiary Limestones of Scinde. Here the spire turns 



round a very elongated axis, so that the shell has almost the form of a 



cylinder drawn to a point at each extremity. Its surface shows a series 



of longitudinal lines which mark the principal septa; and the bands that 



intervene between these are marked transversely by lines which show the 



subdivision of the principal chambers into ' chamberlets.' The chamber- 



lets of each row are con- 



nected 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 multi- 



ple pores in the principal 



septa, such as constitute 



the external orifices of 



the last-formed series, 



seen on its septal plane 



at a, a. 



466. The highest de- 

 velopment of that cycli- 

 cal plan of growth which 

 we have seen to be some- 

 times taken-on by Orbi- 



rmlirm i frmnrl in 

 LUlina,, IS L 



Simple disk of Orbitolites complanatus, laid open to show its 

 interior structure :-a, central chamber; 6, circumambient cham- 

 a type Which, ber, surrounded by concentric zones of chamberlets connected 



long known as a very with each other by armular and radiatin s passage*,. 

 abundant fossil in the 



earlier Tertiaries of the Paris basin, has lately proved to be scarcely less 

 abundant in certain parts of the existing Ocean. The largest recent speci- 

 mens of it, sometimes attaining the size of a shilling, have hitherto been 

 obtained only from the coast of New Holland, the Fijian reefs, and various 

 other parts of the Polynesian Archipelago; but disks of comparatively mi- 

 nute size and simpler organization are to be found in almost all Foraminif- 

 eral 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 ^Egean. When 



