I \l.WhH i;i\i g DIOMED l.. 77 



older types, such as Cidaris,* Aspidodiadema, Phormosoma, and A.stheno- 

 Boraa, the plates of the t\\" rings are (in Cidaris) often wedged apart l>\ 

 the plates of the anal Bystem, or they form a Bingle ring, the genital 

 and ocular plates being in contact laterally (Aspidodiadema), or the genital 

 and ocular plates may be widely separated by the intercalated anal plates 

 (Phormosoma, Asthenosoraa), while such types as some of the Salenidse, 

 iu which only one of the oculars is in contact with the anal system, are 

 intermediate with the typical recent groups of Desmosticha, such as Cce- 

 lopleurus, Podocidaris, Arbacia, Trigonocidaris, and most of the Echinidse 

 and Echinometradse. 



In the Echinothurise the number of anal plates in the outer ring is 

 greatly increased, and it is not possible to fix accurately the position of 

 the original five radial anal plates (see the figures of Asthenosonia hystrix, 

 Plate XXX. Fig. 9, and of Phormosoma placenta, Fig. 8) except in one 

 case, in one of the youngest specimens of Phormosoma I have had occa- 

 sion to examine and to figure (Plate XXX. Fig. 12). See also Challenger 

 Echini. Plate XII. 8 Figs. 2, 7. 



The only other Echini in which I have been able to trace the position 

 of the five original radial anal plates are, first, Pleurechinus bothryoides 

 (Plate XXXI. Fig. 6), in which they are very prominent, and separated 

 by one interradial plate, forming together an outer anal ring of ten plates 

 as regularly arranged as in the oldest Sea-urchins known. Next, those 

 specimens of Arbaciadaj in which we find, as an exception to the general 

 rule, the anal system covered by five radial anal plates, as in Arbacia 

 Dufresnii (Plate XXX. Fig. 14) and A. nigra (Fig. 13), where we have only 

 rarely either a larger or a smaller number of anal plates. The anal 

 plates of Arbacia t in the youngest stages described appear simultaneously 

 as far as we know, and this is probably also the case in Parasalenia 

 gratiosa (Plate XXXI. Fig. 1). 



In a specimen of Arbacia nigra from Talcahuano, measuring over three 

 and a half inches in diameter, the anal system was composed of five large 

 anal plates, four of them being much larger than the fifth. The inner 



* Eocidaris of the Zechstein is undoubtedly to be considered as a Cidaris. Doderlein has called 

 attention (Jap. Seeig., p. 45) to the peculiar bevelled sutures of the ambulacra! with the interambu- 

 lacral plates in this genus; this is such that we can hardly expect whole tests to be preserved. The 

 structure of Eocidaris is in marked contrast to the imbricating of the plates of the test in Echinothurice 

 and in the Palaechinidas. 



f Revision of the Echini, Fig. 68, p. 7:54. 



