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



greater number of wedges. This outer ring is separated from the inner foramen by an 

 inner ring in which the reticular tissue is very close, as in Simtagocystis, Cystechinus, 

 and Pourtalesia (PL XXXIX. figs. 28, 37), or in which it even fills the whole interior of 

 the shaft, as in Cystechinus and Urechinus (PI. XXXIX, figs. 28, 30), or is in the normal 

 Spatangoids separated from the central foramen by an inner ring of more or less distinct 

 wedges, the continuation of the larger ring, as in Lovenia, Cionohrissus, and Hemiaster. 

 In some of the genera we find the reticular structure reduced to a minimum, as in 

 Echinocrepis (PI. XXXIX. fig. 32), and in Periaster (PL XXXIX. fig. 38). From the 

 examination of a number of genera of Clypeastroids and of Spatangoids, there seems to 

 be far less diversity in the structure of the spines in the genera of these two groups 

 than we find in the Desmosticha. 



Chaeacter of Systematic Affinity of Allied Groups of Echinoidea. 



In endeavouring to trace the affinities of the comparatively small number of fossil 

 and living Echinids, it may perhaps be as well to state numerically what we are trying to 

 do, and to show once for all how futile it must be to carry on the attempts which have 

 become so fashionable of tracing the genealogy of this or that group of animals. Very few 

 are so well known from their comparatively unbroken palaeontological history as the 

 Echinoidea and none are at the same time limited to so comparatively small a number 

 of species, both fossil and recent. In order to limit the problem still more, we will take 

 it for granted that we may neglect withm each genus specific difi'erences as not affecting 

 the case, and take alone the fossil and recent genera, which we will assume to be for 

 the present not more than 225, represented by 200U fossil, and less than 300 recent 

 species. 



The genera of the present epoch — say 107, with 300 species — represent the possible 

 combinations of but a small number of Echinoidea, taking into consideration the number 

 of terms which are variable, which are (in a general way only, of coarse) : — the apical 

 system, the actinal system, the genital plates, the ocular plates and the anal plates, the 

 coronal plates, the ambulacral and interambulacral areas, the poriferous zone, the primary, 

 secondary, and miliary tubercles and then- corresponding spines, the modifications of the 

 poriferous zone near the apical and actinal systems, and on the test ; the fascioles, the 

 jaws, the alimentary canal, the position of the apical system, of the anal system, of the 

 actinostome, and the modifications of the same. "We wiU say twenty variables which 

 may be, of course, combined in all possible ways one with the other, and which are 

 capable in their most restricted limits of at least 2^' combinations ; and when we re- 

 member that in the 225 genera which we have thus far recognised, we may imagine 

 any one or all the twenty variables affecting the relationship of each of the genera, it 

 seems somewhat hazardous, to say the least, to attempt anything beyond the broadest 



