712 ON CONCRETIONS, SPICULES, ETC. [ch. 



adsorptive energy may extend throughout the intervening walls. 

 This happens in not a few Radiolaria, and in a certain group called 

 the Nassellaria it produces geometrical forms of peculiar elegance 

 and mathematical beauty. 



When Plateau made the wire framework of a regular tetrahedron 

 and dipped it in soap-solution, he obtained in an instant (as we 

 well know) a beautifully symmetrical system of six films, meeting 



/^JvVvi 



Fig. 327. 



Fig. 328. . A Nassellarian skeleton, Callimitra agnesae Hkl. 

 (0-15 mm. diameter). 



three by three in four edges, and these four edges running from the 

 corners of the figure to its centre of symmetry. Here they meet, 

 two by two, at the Maraldi angle; and the films meet three by 

 three, to form the re-entrant soHd angle which we have called a 

 "Maraldi pyramid" in our account of the architecture of the honey- 

 comb. The very same configuration is easily recognised in the 

 minute siliceous skeleton of Callimitra. There are two discrepancies, 

 neither of which need raise any difiiculty. The figure is not a 

 rectilinear but a spherical tetrahedron, such as might be formed 



