60 PEOr. p. M. DTOCAN A.-SD MR. W. P. SLADEN Olf THE 



The Fifth Basal Plate. — The specimens Discoidea cylindrica 

 in the British Museum show that the fifth basal is not a comple- 

 mentary plate, but 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 Echinoconus alho- 

 galeruSj for that was a part of the left posterior basal. Echino- 

 conus has no fifth basal. 



The Evolution of the Fifth Genital Duct and the Perforation of 

 the Fifth Basal Elate of Species of Discoidea. — Loven has argued 

 that during the lapse of time the generabive organs of species of 

 Eiscoidea became more fully developed after the anus moved out 

 of the apex, and that the fifth plate reaj)peared and became per- 

 forated by a genital duct*. There is much to be 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 cylindrical for instance, have 

 only four perforated basals. D. conica^ Desor, is a G-aulfc 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 Tipper 

 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. JuUieni, 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 appearance of a fifth per- 

 forated basal in time has not been proved to occur in the same 



* Loven, On Pourtalesia, Kongl. Svenska Vetenskaps-Akademiens Handlingar, 

 Bd. xix. No. 7, 1883, p. 68. 



