REPORT ON THE RADIOLARIA. 57 



to the Discoidea, mainly to those which also bear on the periphery of the circular 

 equatorial plane four crossed spines (such as Staurodisculus, Stethostaurus, Staurodictya, 

 &c. ). But in these cruciform Discoidea the shell and the enclosed central capsule are 

 discoidal or lenticular, whilst in the Staurosphserida they remain spherical. Commonly 

 the cross is quite regular, with four right angles and four equal beams ; but often also 

 it becomes more or less irregular. In some genera one pair of equal opposite spines is 

 larger than the other pair. These forms represent the three different axes of the 

 rhombic crystal system, whilst the common regular Staurosphserida represent those of 

 the quadratic crystal system. The latter can be derived from the Cubosphserida 

 (representing the regular crystal system) by reduction of one axis and loss of its pair of 

 spines. In general the number of species (and particularly of the individuals) is much 

 smaller in the Staurosphserida than in all other families of Sphseroidea. 



The Stylosphserida (Pis. 1317) can be derived from the Cubosphserida by 

 reduction of two dimensive axes and loss of two pairs of spines. Therefore, here one pair 

 of spines only is developed, opposite in one single axis. This " monaxonial " form brings 

 the Stylosphserida very near to the ellipsoidal Prunoidea (mainly to many two- 

 spined forms of Ellipsida and Druppulida) ; but they differ from these by the spherical (not 

 ellipsoidal) form of the central capsule and of the enclosing lattice-shell. In the greater 

 part of the Stylosphaerida both spines are of equal size and form, accurately opposite 

 in the " main axis." But in many forms both spines become unequal in size or form, often 

 veiy different. More rarely they are not accurately opposed, but placed in two different 

 axes, intersecting at a small variable angle. The small group of Saturnalida presents a 

 very remarkable and peculiar structure, in which both spines (at equal distances from the 

 centre) are united by a circular or elliptical ring (PI. 13, figs. 15, 16; PL 16, figs. 16, 17). 



The Astrosphserida are distinguished from the other Sphseroidea by the 

 great and variable number of their external radial spines (Pis. 1 1, 18-20, 26-30). Com- 

 monly this number amounts to from twelve to twenty, rarely to only eight to ten, very 

 often to thirty -two to forty or more ; in many species more than one hundred are present. 

 As already mentioned above, it would be important to distinguish between primary spines 

 (as outer prolongations of the inner radial beams) and 'secondary spines (developed from 

 the surface of the shell), but in many cases this distinction is difficult or impossible. 

 More practical is the distinction between larger " main spines " and smaller " by-spines." 

 The size and form of the radial spines is extremely variable. Much more important is 

 their number and disposition. In general we can here distinguish the following different 

 cases : (A) radial spines are developed from all the nodal points of the network on the 

 shell surface ; (B) the number of the spines is smaller than that of the nodal points, but 

 they are irregularly scattered ; (C) the radial spines exhibit a limited number and a 

 certain regular disposition. In this latter case the following modes of distribution seem 

 to be the most important : (a) eight spines placed in the four diagonal axes of the 



(ZOOL. CHALL. EXP. PART XL. 1885.) Rr 8 



