470 



ON CONCRETIONS, SPICULES, ETC. 



CH. 



surfaces, producing a system of concentric spheres. If the siliceous 

 material be not limited to the linear junctions of the cells, but 

 spread over a portion of the outer spherical surfaces or caps, then 

 we shall have the condition represented in Fig. 223 {Ethmosphaera), 

 where the shell appears perforated by circular instead of hexagonal 

 apertures, and the circular pores are set on slight spheroidal 

 eminences ; and, interconnected with such types as this, we have 

 others in which the accumulating pellicles of skeletal matter have 

 extended from the edges into the substance of the boundary walls 



Fig. 22.3. Ethmosphaera conosiphonia, 

 Hkl. 



Fig. 224. Portions of shells 

 of two "species" of 

 Cenosphaera : upper 



figure, C. favosa, lower, 

 C. vesparia, Hkl. 



and have so produced a system of films, normal to the surface of 

 the sphere, constituting a very perfect honeycomb, as in Ceno- 

 sphaera favosa and vesparia*. 



In one or two very simple forms, such as the fresh-water 

 Clathrulina, just such a spherical perforated shell is produced out 

 of some organic, acanthin-like substance ; and in some examples 

 of Clathrulina the chitinous lattice-work of the shell is just as 



* In all these latter cases we recognise a relation to, or extension of, the principle 

 of Plateau's bourrelet, or van der Mensbrugghe's masse annulaire, of which we have 

 already spoken (p. 297). 



