480 MR. HERBERT L. HAWKINS ON 



system. This great increase in the size and extent of the i-ight 

 anterior genital achieves two results. Firstly, the interior of the 

 apical system is filled by it (with the madreporite), and secondly, 

 the posterior margin of the system is completed by its extension. 

 The resulting structure is an apical system of the ethmolysian 

 type (see Gregory, 50). The great importance of this character 

 in Anorthopygus becomes evident when it is realized that such a 

 system is found only in it and in a section of the Spatangidse. It 

 may be stated at once that the method of infilling of the centre 

 of the system shown in this genus is characteristic of all those 

 Holectypoida in which the apical plates regain a genuinely cyclic 

 arrangement. 



The second method whereby the system is rendered compact, 

 without the redevelopment of the posterior genital plate, is shown 

 by Conuhhs. Here the two postei'ior oculars become greatly 

 increased in size, and meet along the posterior margin of the 

 system. The postero-lateral genitals undergo a similar transverse 

 extension to a greater or less degree, and meet above them. The 

 madreporic genital, although large and partly occupying the centre, 

 is in this way separated from the posterior region of the system. 

 A slight an tero-posterior lengthening of the whole system usually 

 accompanies this method of development, and, in a simple sequence 

 indicated in my recent paper, the markedly elongate apical systems 

 of a Pyrina and a Holaster can be readily derived. The Comdus- 

 plan is characteristic of the apical systems of many Jurassic 

 Nucleolitidae, although it is not the only type developed in that 

 complex series of forms. 



Of the type of apical system in which the fifth genital is re- 

 developed (or perhaps replaced by a new but similar plate), 

 Pygaster [Megapygus) shows the first stage. Here one small plate, 

 perhaps more, imperforate and in all probability flexibl)'^ united to 

 the others, makes its appearance at the adapical extremity of the 

 periproct. It seems probable that this new genital plate is a 

 specialized member of the anal series which has become in- 

 corporated into the apical system. In Megapygus it is always 

 small and imperforate. The next stage in recovery is seen in 

 Holectypus sens. str. In this genus the fifth genital is present as 

 a recognizable unit of the genital cycle. It is always smaller 

 than its four associates, however ; and of these, the madreporic 

 genital is very large, occupying all the central part of the system. 

 The posterior genital is still imperforate. In the succeeding series 

 of forms {Cmnholectypus) the relations of the genital plates are 

 similar to those in the earlier subgenus ; but a genital pore, 

 quite as large as those of the other plates, passes through the 

 posterior genital. Co&nholectypus shows, then, the perfect re- 

 storation of the apical system. All five genital glands will have 

 been functional, each with a separate pore ; while the centre 

 of the apical system is filled by the madreporite, situated, as 

 usual, entirely on the right anterior genital plate. 



The apical system of Discoidea is particularly interesting. The 



