IIS' THE JMOEPHOLOGT OP THE CYSTIBEA. 41 



view of these ten plates is the correct one, that is no proof that 

 they did not develop in two rows in Bothrlocidaris as in other 

 Echinoderms. There are many of the later Urchins in which 

 some or all of the oculars come into the border of the periproct 

 and form a closed ring with the basals or genitals ; and they occupy 

 a similar position in certain Ophiurids. But all our knowledge 

 of the comparative anatomy and embryology of these two classes 

 goes to show that this condition is not the primitive one. "Why 

 should it be assumed, therefore, that this was necessarily the case 

 in' the antique Botlvriocidaris ? Are all the conclusions of mor- 

 phology and embryology respecting the fundamental structure of 

 a great subkingdom to be set aside in favour of those deduced from 

 the adult characters of the earliest known fossil member of one 

 of its classes, though by no means the most ancient representative 

 of the subkingdom ? If this be the case, the pjalseontologist will 

 become the absolute arbitrator in all phylogeuetic discussions ; 

 and the results of years of thought and study must at once be set 

 aside if they are not compatible with the characters of a particular 

 fossil, which is liable at any moment to be displaced from its 

 position as the earliest known, and therefore the most primitive 

 type of any group. 



It seems to me that the palgeontologist is here assuming too 

 much ; and as regards this particular case, a curious fact has 

 recently been noted which goes a long way to prove the untena- 

 bility of Neumayr's position. Duncan * has pointed out that in 

 some individuals of Palceecliinus s])liceric.tis the radial plates are 

 intercalated between the genitals (basals) and form with them a 

 ring round the periproct. But, on the other hand, there are other 

 individuals of the same species in which " the five radial plates 

 are triangular and are only intercalated between the basal plates 

 on the outside of the system, and they do not form a part of the 

 ring or margin of the periproct." There is a third condition in 

 which the radial plates are altogether absent, and the basals form a 

 closed ring. This was represented by de Koninck f, whose figure 

 was reproduced by Neumayr, and he proposed to make the 



* "On some Points in the Anatomy of the Species of PalcBechinus (Scouler), 

 McOoy, and a proposed Classification," Ann. & Mag. Nat. Hist. ser. 6, vol. iii. 

 1889, p. 196. 



t " Sur quelques echinodermes remarquables des terrains paleozoiques," Bull. 

 Acad. Sei. Bruxelles, 1869, 2*^ sevie, tome xxviii, p. 545, fig. 1. 



