466 MR. HERBERT L. HAWKINS ON 



regained in the Irregular Echinoids, as in Discoidea and many 

 Clypeastroida, the periproct is so small as to necessitate very 

 little modification of the steady progress of the coronal plates 

 from the apex to the peristome. 



2. The Shape of the Periproct. 



In Pygaster sens, str., the periproct is roughly elliptical in 

 shape, and very large, often having the same width (in transverse 

 measurement) as the apical system. In Alegapygus and Macropygus 

 its outline becomes pyriform, owing to the partial closing in of 

 the interambulacral plates round its adapical extremity. The 

 width never becomes greater than in Pygaster sens, str., but the 

 actual size is much larger in these later subgenera, owing to the 

 backward shifting of the posterior edge of the periproct without 

 a corresponding retraction of the adapical margin. The pyriform 

 shape caused by this lagging behind of the upper part of the 

 periproct leaves its impression on the shape of the aperture in 

 later genera. In Holectyjyus, for example, the periproct has 

 its ad oral margin rounded, but adapically it tapers to a point. 

 The same feature is seen in Pileus. In Holectypus sens, str., the 

 periproct is still large ; in some species, e. g., H. depressus, it is 

 of an extraordinary size. But in Coenholectypus it has generally 

 decreased so as to be smaller than the peristome. In Anortho- 

 pygus the periproct is of moderate size, and has a characteristically 

 oblique position. Obliquity in the case of the peristome is not 

 uncommon among Irregular Echinoids (e. g., Pyrina and Tremato- 

 pygus), but this is practically the only form where such asym- 

 metry affects the shape of the periproct to a considerable degree. 

 In this connection it is interesting to find that in P. [Megcipygtis) 

 timbrella the large pyriform periproct shows a distinct inclination 

 towards the left side of the interradius, thus giving an indication 

 of potential obliquity. 



In Discoidea the periproct is usually lanceolate in outline, 

 often equally pointed at both extremities, but it is always longer 

 than broad. In Comdus the marginal periproct is similar in 

 shape to that of Holectypus hemisphcericus, although smaller in 

 size. The pointed character of its adapical part is more pro- 

 nounced in young specimens than in adults. The size and 

 shape of the periproct in the Echinoneidse compares well with 

 those in the Conulidee. In the Clypeastroida the periproct is 

 always adorally situated, as in Discoidea, but it is veiy small, and 

 usually circular in shape. 



3. The Anal Plates. 



The plating of the periproct-membrane is at present unknown 

 in Pygaster, Pileus, and Comdtos, In the case of the two genera 

 first named, this is probably due to the large size of the periproct, 



