SECT. ii. THE ORBITOLITE. 33 



quite a mass of sarcode. The Dendritinae are inhabi- 

 tants of shallow water and tropical seas, while the other 

 members of the genus Peneroplis abound in the Eed 

 Sea and the seas of other warm latitudes, especially in 

 the zone of the great laminarian fuci. They do not 

 appear in a fossil state prior to the beginning of the 

 Tertiary period. 



The last whorls of some of the compressed spiral 

 Foraminifera of the Porcellanous order so nearly encom- 

 pass all their predecessors, that the transition from a flat 

 spiral to the Orbitolite with its flat disk of concentric 

 rings is not so abrupt as might at first appear. The 

 gradual change may be distinctly traced in the species 

 of the genus Orbiculina. The exteriors of the shells of 

 the genus Orbitolites have less of the opaque whiteness 

 than many others of its family. In its simplest form it is 

 a disk about the 5-^ of an inch in diameter, consisting 

 of a central nucleus surrounded by from ten to fifteen 

 concentric circular rings. The surface is usually plane, 

 though sometimes it is concave on both surfaces in con- 

 sequence of the rings increasing in thickness towards 

 the circumference. The rings or zones are distinctly 

 marked by furrows on the exterior of the shell, and each 

 of these zones is divided by transverse furrows into ovate 

 elevations with their greatest diameter transverse to the 

 radius of the disk, so that the surface presents a number 

 of ovate elevations arranged in consecutive circles round 

 the central nucleus. The margin of the disk exhibits 

 a series of convexities with depressions between them ; 

 in each of these depressions there is a circular pore sur- 

 rounded by a ring of shell: these pores are the only means 

 the animal possesses of communicating with the water 

 in which it lives. 



Fig. 98 is a horizontal section of the simple Orbitolite 

 showing the internal structure of the disk. A pear- 

 shaped chamber with a circumambient chamber forms 



VOL. II. D 



