REPORT ON THE RADIOLARIA. 719 



in the Tropic of the Capricorn). In the figures of the Pis. 131-140 (as well as in 

 my Monograph, 1862, Taf. xv.-xxii.) the four northern or canceral spines are 

 marked by the characters 61 to 64, and the four southern or capricornal spines by the 

 characters dl to cZ4. Also the eight tropical spines lie (crossed in pairs) in two 

 meridian planes ; they do not lie, however, in those perradial planes, in which are placed 

 the twelve other spines ; but in two different meridian planes, crossing the former 

 at angles of 45; we call these the "secondary" or " interradial " meridian planes. 

 Each of these planes is determined by the spineless axis and by two crossed inter- 

 radial or secondary axes ; in each of the latter lie two opposite tropical spines. 

 In the first interradial meridian plane lie 61 and 63, dl and J3, in the second 62 

 and 64, d'2 and tZ4. 



It is a most interesting and important fact, that in all Icosacantha (A c a n t h o- 

 n i d a and A c a u t h o p h r a c t a) this regular disposition of the twenty spines (in five 

 parallel zones and four meridian planes) becomes constantly preserved by heredity, 

 whilst the form and size of the different spines are extremely varied by adaptation. 



Only in a minority of the Icosacantha are all twenty spines perfectly equal or nearly 

 equal in size and form ; and then it is often very difficult to distinguish the different 

 zones in their disposition. But in far the greater part the size or the form of the twenty 

 spines becomes different in different zones ; and then we can commonly distinguish 

 easily the five different zones. Firstly, in all Quadrilonchida and Dorataspida, the four 

 equatorial are distinguished from the sixteen other spines either by form or by size, and 

 often in a very remarkable degree. As soon as these four principal spines are recognised, 

 it is easy to determine also the sixteen others ; for the eight polar spines lie in the same 

 two (perradial) meridian planes as the former, whilst the eight tropical spines lie in 

 two different (interradial) meridian planes, intersecting the two former at angles of 

 45. Commonly, therefore, this distinction is rather easy. 



In the majority of the Icosacantha all four equatorial spines are exactly of the same 

 form and size. But in four families the two opposite spines of one equatorial axis are 

 much larger, or of another form, than those of the crossing axis. This is the case in the 

 Amphilonchida, Beloriaspida, Hexalaspida, and Diploconida. Therefore we here call the 

 major equatorial axis (with larger spines) the " hydrotomical axis," and the minor axis 

 (with smaller spines) the " geotomical axis." Correspondingly, the meridian plane, in 

 which the two larger equatorial spines are placed (cl, c3) and the appertaining four polar 

 spines (al, a3, el, <?3) may be called the " hydrotomical plane "; in the remarkable family of 

 Hexalaspida (PL 139) all six spines of this hydrotomical plane are much larger than 

 the other fourteen. Perpendicular to this plane is the second perradial meridian plane, 

 which we call the " geotomical plane " ; in it lie the two smaller equatorial spines (c2,c4) and 

 the corresponding four polar spines (a2, a4, e2, e4). In some Hexalaspida (Hexonaspis 

 and Hexacolpus) the six spines of the hydrotomical plane become so preponderant that 



