50 
conica here reproduced (Fig. 10). The only natural explanation is, 
that it is the madreporic plate which may be more or less widened 
towards the middle of the apical system. Of a central plate there 
is n0 trace in the apical system. In the anal system there seems 
to be no distinct suranal plate either, so far as evidence goes from 
the little we know of the plates covering the periproct in this 
group of fossil Echinoids. — Lambert in the paper ,,Sur un 
Discoides inferus (Desor) recueilli å Tancarville (Seine-Inférieure) 
(R. Fortin. Notes de Géologie Normande. VI. 
Bull. Soc. géol. Normand. XVIII, p. 20—22) 
certainly names the two small plates in the middle 
of the periproct (Fig. 11) the central plates 
(sometimes there is only one such plate). But it 
is evident that this ,,central" plate is not the 
homologue of the central or suranal plate of Sa- 
Fig. 11. Anal 
system of Dis- 
coidea infera. . proctal plate, though scarcely originating through 
After 
lenids and Echinids; it is simply a small peri- 
»la fissiparité des valvulaires contigués" (those 
Lambert). 
plates surrounding the anal opening), as suggested 
by Lambert. If any homologue to the suranal plate were to be found 
here, it could only be the large one at the upper end of the periproct, 
the ,,0percular" of Lambert; but as this plate is stated by 
Lambert to be sometimes wanting, it is evident that no special 
morphological value can be ascribed to this plate either. — The 
conclusion that the Holectypoidea have no central or suranal plate 
is perfectly in accordance with their close relation to and un- 
doubted descent from the Diadematoidea, which likewise lack the 
suranal plate. 
In the Clypeastroidea the plates of the apical system are 
so completely coalesced that it is quite impossible to state anything 
about the presence or lacking of a central plate from the examina- 
tion of the grown specimens. The study of the development 
of the plates can alone give the clue to the question whether there 
