IN THE MUD OF THE LEVANT. 51 



internal organisation may separate into two parts, 

 according to Ehrenberg's definition, but that the 

 subsequent formation of the two transverse sili- 

 ceous septa is owing to the performance of some 

 act vaguely analogous to secretion, concern- 

 ing which little is really known — each portion of 

 the endochrome closing up its own half of the 

 original cell, before these separate. Indeed, in 

 many species, two new and distinct frustules form 

 within the interior of a larger one, and are 

 liberated only by the breaking up of the latter. 

 This is what we see in Isthmia and Biddulphia, 

 and is manifestly distinct from the " mere tension" 

 of Ehrenberg. 



Whatever may be their nature, the siliceous 

 cases of these organisms occur in oceanic deposits 

 from almost every part of the globe. In the 

 Levant mud however, they are not very numerous, 

 when compared with other structures. As is 

 well known, they occur in the greatest perfection 

 and abundance in the deposits of Bermuda and 

 Virginia, — fields, in which Dr. Bailey and his 

 American coadjutors have won such lasting fame. 

 Fig. 23 represents a small species of Campylo- 

 discus. 



