1690 THE VOYAGE OF H.M.S. CHALLENGER. 



tabulate, panelled or dimpled. Central capsule excentric, placed in the aboral half of 

 the shell-cavity. 



The family C i r c o p o r i d a comprises those Ph^odaria in which the simple, 

 spherical, or polyhedral shell exhibits a peculiar porcellanous structure, possesses a 

 simple mouth, and is particularly distinguished by the constant character denoted by their 

 name, viz., by cii'cles of pores surrounding the base of the radial spines. It agrees in 

 •these striking peculiarities with the following family, the Tuscarorida, but differs from them 

 essentially by the spherical or polyhedral form of the shell, which is never ovate, with the 

 main axis prolonged. The radial spines, variable in number, are regularly or symmetrically 

 disposed in all Cii'coporida, whilst they are arranged around the poles of the main axis 

 in all Tuscarorida. Whilst the latter are always extremely large, more than a 

 millimeter in size, the former are much smaller and never reach the diameter of a 

 millimeter. The number of genera and species in this family is not large, and the 

 majority are very rare, but they belong to the most remarkable Eadiolaria l^y the 

 peculiar structure, as well as by the geometrical regularity of the polyhedral shell. 



The Circoporida may be divided into two different subfamilies, the Circogonida 

 and the Haeckelinida. The latter are represented by the single genus Haechdiana 

 (PI. 114), and possess a spherical shell with numerous small dimples between the 

 circles of pores, and numerous simple radial spines which do not exhibit a regular ar- 

 rangement. The Circogonida, on the other hand, comprise all other genera (Pis. 

 115-117), and possess a panelled shell with polygonal plates, and a certain number 

 of radial spines, which are regularly disposed on the corners of the polyhedral shell, 

 and usually armed with verticillate bristles and terminal forks or spathillse. Perhaps 

 the Circogonida and Haeckelinida may better be considered as two separate families. 



The geometrical fundamental form of the shell is in the Circoporida of the highest 

 interest, since it exhibits regular proportions, which are very rare in other organic 

 forms, generally some rare forms of regular jjolyhedrons. The shell in all members of 

 this family belongs to that group of geometrical forms which I have called in my 

 General Morphology " Pohjaxonia" (vol. i., 1866, pp. 406-416). The shell therefore 

 is constantly an " endospherical polyhedron," i.e., a polyhedron, all the corners of which 

 lie in a spherical surface. The corners are always indicated by the bases of the radial 

 spines aiising from the surface of the shell and marking its axes, and it is immaterial 

 whether the shell itself is a true polyhedron or a sphere ; for in the latter case also the 

 radial spines mark the axes of the polyhedron. 



The six genera, which we here distinguish among the Circoporida, represent six 

 different fundamental forms. Clrc02Mrus (PL 115, figs. 1—3; PL 117, figs. 5, 6) 

 possesses six radial spines, opposite in pairs in three diameters, perpendicular one to 

 another; it agrees therefore with the Cubosphserida (p. 169, Pis. 21-25) and rej^ire- 



