12 H. L. Hawkins — Anneal System in the Eoledi/poida. 



Macropygus) the structure is similar to that obtaining in P. xunhrella. 

 Indeed, it is natural that this should be the case, for the shape of the 

 pei'iproct is similar in both forms. 



Pygader, sens, sir., and Maoropi/yns show an attempt to reconstruct 

 the cycle of the genitals by the development of one or more plates in 

 the posterior part of the system, while the centre of the system is 

 filled up by the intrusion and expansion of the madreporite. 



D. The Apical System of ANORTnopraus. (PI. II, Tigs. 3, 4.) 



Although contrary to the true stratigraphic sequence, it seems best 

 to discuss this genus directly after Pygaster, as its apical system has 

 more pronounced affinities with the Pygaster type than with that of 

 its associates in the Lower Cretaceous. In all but the tuberculatiou 

 and the position of the periproct it is a Pygaster, and was classed as 

 such long after its speciho recognition. 



Cotteau (1859 and 1861) gives good figures of the apical system in 

 both species of the genus. There are several siliceous moulds of 

 A. ort)icularis, from the Haldon Hill drift, in the British Museum, 

 and they serve to verify the drawings taken from Continental 

 specimens. 



The system is a complete cycle, and, as far as its outline is 

 concerned, is almost radially symmetrical. In spite of this appearance 

 it is in reality of quite unsymmetrical structure. There are only four 

 genital plates, the posterior one being absent. Three of the remaining 

 plates are of uniform size, but the madreporic plate is so extended as to 

 render the system actually ethmolysian. In A. orbicularis (PI. II, Fig. 4) 

 this large development of the right anterior genital is not carried out 

 at the expense of the size of its fellows, as it is in Pygaster, for it is 

 very little wider than the other three genitals. The posterior part 

 of the plate has the outline of a perfectly normal fifth genital, and 

 appears sharply marked off from the central (madreporiform) part of 

 the plate. There is, however, no trace of a suture. The second 

 species, A. michelini (PI. II, Fig. 3), has, accoi'ding to Cotteau, a much 

 more Pygasfer-like apical system. In it the madreporite is expanded 

 transversely as well as posteriorly, and reduces the three other genitals 

 to dimensions hardly greater than those of the oculars. These last 

 are all five similar in form and size. 



A feature of the system in A. orbicularis that appears in the moulds 

 is the unequal thicknesses of the plates. The impressions of the 

 oculars, and of the left anterior genital, are all shallow — in fact, at 

 the same level as those of the coronal plates. But the other three 

 genitals (the two postero-lateral and the madreporic plates) have left 

 very much deeper scars on the mould, indicating their greater thickness. 

 All the five specimens I have seen agree in this point, which, although 

 of unknown significance, seems worth noting from its persistent 

 occvirrence. 



In Anorthopygus, then, the cycle of the apical system is complete 

 in form, but the posterior genital is missing, and the centre of the 

 system is filled by the madreporite. 



