H. L. Hawkins — Apical System in the Holectypoida. 11 



certain that the fifth (posterior) genital did not exist. At most it 

 may have been represented by one or more small plates situated on 

 the anal membrane, and not necessarily distinguishable from the 

 other anal plates. We find, then, that the first effect of the shifting 

 of the periproct was the actual destruction of the odd genital 

 plate, and the spreading out of the rest of the system into a trans- 

 verse line of plates. The madreporie genital is larger than its 

 fellows, but not very much so, and, as the whole system is not 

 a cycle, it cannot be said to intrude at all into the centre of the 

 apical system. 



In ' Plesieohinus ', then, the system is absolutely ruptured, with no 

 attempt at repair, while there is equally no trace of any infilling of 

 the central part of the ' disc '. 



2 Pi/gaster, sens. sir. The apical system is well known in 

 F. umhrella, P. dilatatus, and P. macrocyphus. Cotteau (1874) figures 

 the system in all three species, and I have a specimen of P. umhrella 

 in my collection in which most of the plates are well preserved. In 

 this group, owing to the closing in of the upper part of the inter- 

 ambulacrum upon the adapical portion of the periproct, it is possible 

 for the apical system to begin to reassumeits cyclic character. In the 

 work just referred to Cotteau seems to assert the presence of five 

 perforated genitals in the genus, but this is certainly a mistake. 

 (His figures give no support to his assertion.) Pomel (1883) is nearer 

 the truth in his statement that the place of the fifth genital is taken 

 by several small plates. I have not seen any specimens or figures 

 which show any such plates preserved, but, judging from the facets 

 on the posterior margins of the plates in my specimen, it seems 

 certain that at least one plate occupied the position of the posterior 

 genital. This will have been a roughly pentagonal plate (marked 

 A in PI. II, Fig. 2), in contact with the madreporie genital and the 

 right posterior ocular, situated somewhat to the right of the antero- 

 posterior axis of the system. To complete the symmetrical outline of 

 the system it is necessary to postulate the presence of one or more 

 smaller plates besides this one. Their probable positions are indicated 

 in PI. II, Fig. 2, by dotted lines. 



The most striking feature of the apical system in this group is the 

 enormous size of the madreporie genital. Not merely is it wider and 

 more convex superficially than the other genital plates, but it extends 

 inwards and posteriorly through the system, entirely separating the 

 other genitals from one another, and monopolizing all the central 

 space. In fact, the madre]3orite occupies just the position which, in 

 a Eegular Echinoid, would contain the anus and its membrane. The 

 genital pores are relatively small, and each is encircled by a prominent 

 raised rim. The ocular plates are small, and are rendered the less 

 cons];)icuous by their depression below the general surface-level of 

 the surrounding plates. They slope away from the centre, so that 

 the ocular pore opens in a considerable hollow. After the pore is 

 passed, however, the surface of the plate rises sharply to meet the 

 proximal coronal plates. 



Judging by Cotteau's figures of the apical system of P. trmicatus, it 

 would seem that in that species (the type of Pomel's third ' genus ' 



