SKA-URCUTXS. 4(i7 



and tl\o resulting flexibility of the niembinne nnd weakne^^s of 

 the plating. In the case of Couul-us (and also of those I/olecti/pl 

 which have the periproct rn;ii-giiial), the exposed position of the 

 anal plates on the ambitus may account for their non-prevservation. 

 A thickly plated membrane occupies the periproct of Echbionetis. 



In Discoidea the plates of the pei'iproct are not infrequently 

 found in situ, and they are known in Caen/wl^ctij^^cs , Anortho- 

 pj/giis, and GoptocUscus. The last-named genus differs, a*; I'egards 

 this character, from all the others, in possessing a single ring of 

 almost equal-sized plates around the inner margin of the 

 periproct, and in not having, so far as is known, any smaller 

 plates in the immediate surroundings of the azius. 



DisTohlpAt has one large anal plate, usually bearing a tubercle, 

 occupying most of the adoi'al half of the peiipioct-opening, and 

 a series of fringing plates which decrease in size as they approach 

 the adapical part of the aperture, A few, often only two, small 

 plates occur witiiin this irregular ring, and they are always in 

 contact with the largest plate. The anus is thus situated quite near 

 to the adambital edge of the periproct, in a position far removeil 

 from the mouth. In CoenhnJeGtyjnis, to judge by a figure of 

 G. juUieni from Algiers (Peron &, Gauthier, 34), the arrange- 

 ment was on a similar plan. There, however, the adorally 

 situated plate is relatively small, and the fringing plates are also 

 smaller and more numerous than in Discoidea. The inner anal 

 plates are exceedingly minute, and are presei-ved in considei-able 

 numbers. 



In Anort?iop}/gus I have been able to study only the outlines 

 of the anal plates, these being easily traceable on a siliceous 

 mould of J. orbicularis in the British Museum, In this specimen 

 the arrangement of the plating is exactly the reverse of that 

 which obtains in the two genera just described. The largest of the 

 anal plates are adapically situated in the oblique periproct, and 

 a series of pentagonal and hexagonal plates, of approximately 

 equal size, covers all the remaining smface of the aperture 

 except for a very small area in its extreme adoral part. The 

 actual anus, which is I'epresented in the mould by a. pi'ominent 

 unsutured portion of the infilling matrix, lies in the true antero- 

 posterior axis of the test, thus being unaflijcted by the asymmetiy 

 of the periproct as a whole. There seems to have been no space 

 occupied by plates between the anus and the peripi'oct mai'gin. 



The position of the anus, in its relation to the situation of the 

 periproct on the test, is interesting. When the aperture is on 

 the adoral surface the anus tends to open in its adambital corner, 

 while the same tendency, with an opposite efiect, appears when 

 the periproct is supramarginal. A generalization, founded on 

 the somewhat slender evidence of only three generic types, may 

 be made that: — Wherever the periproct may be situated, the 

 anus assumes a position within its borders as near to the ambitus 

 (i. e., the most posterior part of the te.st) as possible. 



30* 



