SEA-tJROniNS. 403 



C. T h (' P e r i p r o c t. 



Since the exoentric position of the periproot, outside the .apical 

 cycles of [)lntes, is a diagnostic feature of the Irregular Echinoids 

 MS a. whole, it is natural to find that in the Holectypoida, which 

 includes the most primitive of the " Exocyclie" forms, its position 

 is very variable. When once the periproct has left the apex, its 

 chief tendency seems to be to assume a position as absolutely 

 posterior as possible, and in the course of its passage to such a. 

 position, it undergoes many changes itself, and is the cause of 

 many othei's to the test. It always lies in the postei'ior intei"- 

 aml)ulacrum. 



1 . Tlie Position of the Periproct. 



Practically the only distinguishing feature between a young 

 specimen of a Pi/gaster sens. .str. and a primitive Diademoid is 

 the fact that in the former the periproct has broken through the 

 posterior part of the apical system. It cannot be said to lie 

 altogether outside the system, for to some extent it occupies the 

 position of the posterior genital plate, and extends well up to the 

 apex of the test. It is, in part, more nearly central in position 

 than in some of the 8aleniida^, although its large size causes it to 

 reach away fi-om, as well as into, the apical system. 



In Pygasier sens, lat., the periproct is always in contact with 

 the apical system, — in the earliest forms reaching to the inner 

 margins of the anterior and antero-lateral genitals, and in the 

 later ones touching only the outer margins of the redeveloped 

 posterior genital. When traced from Pygaster sens. str. to 

 ALccropygus, however, the position of the widest part of the 

 periproct is found to pass gradually backwards, while the posterior 

 edge of the opening approaches the ambitus of the test. Pygaster. 

 then, shows a .stage in which, although the periproct retains 

 its primitive associa,tion with the apical system, the posterior 

 tendency in its position is recognizable. 



Pileus and Anorthopygus agree in having the peripi-oct entirely 

 on the adapical surface of the test, but quite separated from 

 the apical system. In most cases, however, the posterior part 

 of the periproct is not so near to the ambitus as in Macropygus. 



In Holectypas we find two groups, as regards the position of 

 the periproct. Both groups appear a.t almost the same stage of 

 the Lower Oolite, but one is more retarded in character than the 

 other. The former, which may be exemplified by H. hemi- 

 sphcericus, has the periproct opening on the margin, that is, in 

 the posterior extremity, of the test. The latter, of which a 

 common representative is //. depressus, has the periproct entirely 

 on the a-doral surface, and often very close to the peristome. It 

 would appear a.t first sight that in the former group the periproct 

 hail i-eached the necessary limits of its retrogression, and in the 

 latter had, as it were, overshot the mark. Rut, in tlie liglit of 



