14 II. L. Rairkin-s — Apical Sf/stem in the Holectt/poida. 



I hope to show in a future paper, is near the parting of the wars 

 between some important lines of evohition, and tlie unstable characters 

 of its apical system are indices of that fact. 



The cycles "of plates are always complete, the genitals being nsually 

 tumid and considerably larger than the oculars. Whether the 

 madreporite is in its usual plate, or scattered over all five, the right 

 anterior genital is never much larger than the other four. All genitals 

 take an equal share in the occupation of tlie centre of the system. 

 This uniformity in size of the genitals confines the oculars to 

 a marginal position in the system, and, owing to their relatively 

 small size, they play quite an unimportant part in the construction of 

 the apical disc. 



In Disco'idea, then, the cycle is complete, and the centre of the 

 system is occupied by equal extensions of all five genitals. The 

 madreporite may, or may not, be central. 



G. The Apical System op Conulus. (PI. II, Fig. 8.) 



There is a curious diversity of opinion as to the composition of the 

 apical system in this genus. Wright and Cotteau both describe it as 

 having five genital plates, the posterior one being small and imperforate. 

 This small plate would, by its presence, serve to distinguish Conulus 

 from Pi/rijia (see PL II, Fig. 9). In spite of the figures given by 

 these authors I have been unable to find any trace of a fifth genital, 

 although I have examined several hundreds of specimens of the 

 common Eritish species. Even in cases of marked abnormality no 

 plate in the position of the posterior genital has been distinguishable. 

 It seems unlikely, however, that both these authorities can have 

 been mistaken, so that it is probable that some specimens (of both 

 C. sichrotundus and C. albogalerus) possess the posterior genital. 

 I think it is safe to assume this character to be exceptional, and 

 that the great majority of individuals have only the four genitals. 

 I shall only describe and figure here the type that I have seen. 



The apical system is not quite circular, but tends to be elongated 

 along the antero-posterior axis. The tendency is, however, very 

 slight. The four genitals, taken together, build a roughly square 

 figure, into the middle of three of the faces of which the 

 anterior oculars are inserted. The madreporic genital is considerably 

 larger than the others, and occupies most, but not all, of the centre 

 of the system. It never quite separates the two postero-lateral 

 genitals. The madreporite is normally restricted to the right anterior 

 plate. (There is a specimen of C. albogalerus in the collection of 

 .Mr. LI. Treacher, F.G.S., which, in addition to other slight 

 abnormalities, has the madreporite on all four of the genitals.) In 

 the absence of the fifth genital the two posterior ocxdars meet along 

 tlie median line, and are of much greater size than the other three. 

 They are sometimes of about the size of the non-madreporic genitals, 

 but are usually smaller. 



Conulus, then, shows the cycle completed by the large development 

 of the posterior oculars, while the centre of the system is largely, 

 though not entirely, occupied by the madreporite. 



