10 H. L. Hau-kina — Apical System in the Holedypoida. 



will have to become a compact group of plates instead of a hollow 

 ring enclosing a membrane-covered space. 



There are thus two featui-es in which the evolution of the apical 

 system of the ])rimitive Im-gular Ecliinoids may be profitably traced. 

 The first is the recovery of the complete cycle of plates, and the second 

 the method of the infilliny of the centre of the system. 



Discussions and brief descriptions of the structure of the apical 

 system in typical genera will now be given, and subsequently the 

 stages in its evolution will be summarized. 



C. The AricAL System: of Pygaster. 



As far as the cliaracters of this structure are concerned, the species 

 of Pyyaster fall into two well-marked groups. These groups correspond 

 to the divisions named by Pomel (1883) '■ Plesiechinus^ ^ and Pyyaster 

 [sens. strr\. The differences in the apex are due to the different 

 shape of the periproct in the two divisions. In the ' Plesiechimis ' 

 section the periproct has its greatest width in the adapical portion,, 

 while in Pyyaster, sens, str., the upper part is narrow, and the 

 greatest width of the periproct is about midway between the apex 

 and the margin of the test. ' Plesiechiniis^ comes first both in 

 stratigraphical occurrence and in phylogenetic order. P. macrostoma 

 (Wright) is the only species of the group in which the apical system 

 seems to be known. This species was suspected by Wright himself 

 (1860) to be onlj' a variety of Pyyaster ' {Plesiechimis)'' semistdcatus, 

 and it may at least be taken that the apical structures were similar 

 in both forms. ' Plesiechitms ' may be said to contain those species 

 in which the periproct has only just broken through the apical 

 system, or even is in the act of breaking through. In Pyyaster, 

 sens, str., it has, for the most ])art, receded from the apex, and 

 the sides of the interambulucrum are closing in to heal the gash 

 caused by its passage. Pileus marks the complete accomplishment of 

 this process. 



1. 'Plesiechimis' (PL II, Fig. 1). In the Supplement to his 

 Monograph, Wright (1860) figures the apical system of a small 

 individual of P. "■ [Plesiechimis)' macrostoma (pi. xli, fig. 5h, by error 

 termed meyastoma in the legend). The drawing (copied in PL II, 

 Fig. 1, of this paper) shows the apical plates to be arranged around 

 the upper edge of the very large periproct and to form a smooth, 

 apparently complete, margin to the aperture. Only four yenitals are 

 present, arranged in an almost linear manner transversely to the 

 diameter of the test. Of the five oculars, the three anterior plates 

 are normally situated in the angles of the genitals, while the twO' 

 posterior plates rest against the posterior margins of the two postero- 

 lateral genitals. In view of the fact that the posterior margin of the 

 system is regular, with no considerable re-entrant angles, and that 

 the posterior oculars do not reach to the periproct margin, it seems 



^ This name is systematically indefensible, being synonymous with Agassiz's 

 Pygastcr, whose type is P. semisulcatus. I use it in inverted commas here to 

 avoid systematic discussions in a morphological paper. 



