less regular in size, and some in which the hexagonal meshwork has been developed not only on one outer surface, but at successive {470} 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. 223. Ethmosphaera conosiphonia , Hkl. Fig. 224. PorôÙtions of shells of two ãspeôÙciesã of CeôÙnoôÙsphaeôÙra : upôÙper figôÙure, C. favôÙoôÙsa , lowôÙer, C. vesôÙpaôÙria , Hkl.
and have so produced a system of films, normal to the surface of the sphere, constituting a very perfect honeycomb, as in Cenosphaera favosa and vesparia78.
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