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



former uniform extension of the ambulacral pores as far as the actinostome. In the 

 structure of the apical system, the subanal plate can still be traced in some of the stages 

 of growth, while in Temnopleimis it never becomes entirely obliterated. In Temno- 

 pleurus we might say the Salenid abactinal system was more readily traced, and still 

 better in Arbacia, while in both these genera the Cidarid features of large primary 

 tubercles is retained in a different degree, and in Arbacia, in one part of the ambulacral 

 zone, the arrangement of the pores is of an ancient type, while towards the actinostome 

 its petaloid structure is eminently recent ; the structure of the ocular plate of Arbacia 

 leads us back directly to the structure of the ocular plate in the oldest Palseechinidse. 



The affinities of the Clypeastridae with the Discoidese are clearly indicated by the 

 development of the longitudinal axis, which dates from the exclusion of the anal from 

 the apical system. We readily trace through Pileus, Holectypus, and Discoidea affinities 

 to Galerites and the fossil Conoclypeidse, while with the appearance of Galerites, 

 Fihularia and Echinocyamus we have the element of the Clypeastridae and Scutellidse ; 

 and their relationship to the Cassidulidse is well shown in the simple ambulacral system 

 of some of the genera, and the rudimentary auricles still to be traced among the 

 Echinolampadse, while the affinity of the earliest CassiduHdas, Hybodypus, Galerop)ygus, 

 and the Uke, to Pygaster, which culminate in our day with but slight modifications in 

 Echinoneus, show how clearly related the earlier Spatangoids were with the genera to 

 which the Clj-peastroids are most closely related, which in their turn still show a most 

 unmistakable relationship to the Desmosticha, so much so that it seems difficult to say 

 whether some of the Echinolampadae of the present day are not more closely related to 

 the Galeritida3, from the slight development of the petaloid system and the presence of 

 jaws or of rudimentary auricles. 



Already, in the Jura, Pygaster shows the method of the passage of the anal system 

 from the interior of the anal ring to the odd interambulacral space, and we find genera 

 such as Holectypus and Discoidea, in which it occupies in succession all possible positions 

 between the apical system and a place close to the actinostome ; and the passage once 

 effected in the Clypeastroids, we readily go from a mere circular or elliptical opening placed 

 either in the axis, or obliquely or transversely to it, to an opening in a slight groove or 

 a more or less deep groove occupying this same odd interambulacral space, having its 

 climax in the Echinobrissinse, and then we most naturally pass to an opening holding 

 a certain relation to a more or less distinct beak which, combined with the subanal 

 plastron enclosed by the subanal fasciole, we can gradually trace from a simple plastron 

 flush with the test, as in the earliest Holasteridse, to the Echinocardise, to the Brissinse, and 

 finally to the Pourtalesise, to a plastron extending as a slight beak below the anal system, 

 and finally forming a more or less distinct snout ; and when this is combined with the 

 deep anal groove of other Spatangoid genera we get the remarkable forms such as we 

 have described as belonging to the Pourtalesise. 



