60 PROE. P. M. DUNCAN AND AIR. W. P. SLADEN ON THE 
The Fifth Basal Plate. — The specimens of Discoidea cylindrica 
in the British Museum show that the fifth basal is not a comple- 
mentary plate, hut a true basal which is not perforated for a 
genital duct, but which is penetrated by the madreporite. The 
position and dimensions of the plate are those of a normal basal 
in other forms ; and it is not comparable with the so-called fifth 
plate described by Cotteau in one specimen of Fchinoconus alho- 
galerus, for that was a part of the left posterior basal. Fchino- 
conus has no fifth basal. 
The Fvolution of the Fifth Genital Duct and the Perforation of 
the Fifth Basal Plate of Species of Discoidea. — Loven has argued 
that during the lapse of time the generative organs of species of 
Discoidea became more, fully developed after the anus moved out 
of the apes, and that the fifth plate reappeared and became per- 
forated by a genital duct*. There is much to he advanced in 
favour of this remarkable generalization, and it is certainly the 
case that the oldest species had four basals perforated by genital 
ducts, whilst the youngest had five perforated basals. The 
oldest species do not, however, obtain a fifth genital duct, and its 
perforation during lapse of ages does not occur ; for the oldest 
and youngest forms of Discoidea cylindrica , for instance, have 
only four perforated basals. D. conica, Desor, is a Gault and 
Albien species of Europe and Africa, and it follows Loven’s law, 
and has but four basals perforated by the duct, and the fifth is 
imperforate. D. subuculus ranges from the Warminster Upper 
Greensand into the Lower Chalk ; and it has been described as 
having only four or sometimes five basals perforated ; there 
are two specimens in the British Museum in which all the basals 
are perforated. 
Discoidea minima , D. Favrina, D. Jullieni , and D. Forgemolli 
are European and North- African forms, and all have five basals 
perforated ; and the age of the fossils is Cenomanien. D. infera 
and D. Dixoni are from the Upper Chalk, and all the five basals 
are perforated. 
So far as the whole genus is concerned, the generalization of 
the appearance of the fifth basal perforation in the later ages of 
its lifetime is proved ; but the appearauce of a fifth per- 
forated basal in time has not been proved to occur in the same 
* Loven, On Pourtalesia, Kongl. Svenska Yetenskaps-Akademiens Handlingar, 
Bd. xix. No. 7, 1883. n. 68. 
