22 PEOF. p. M. DUKCAlSr's HE VISION OF THE 



regarding the internal branchiaB of the Echinothuridse and 

 Ludwig's remarks concerning the rudimentary nature of these 

 organs in some Diadematidse, coupled with the knowledge that in 

 some genera of this great family the internal branchise are not 

 seen, renders it impossible to classify upon the old grounds- 

 The difficulty is where to place the Echinothurid^, and the Tem- 

 nopleuridse, Echinometridae, and Echinidse in reference to the 

 Diadematidse and close allies, and the Saleniidse. It is the opinion 

 of A. A gassiz that the Echinothuridae should be placed close to 

 the Diadematidse and not with the Palseechinoidsea ; and it ap- 

 pears to be justified by facts. But the flexible forms with the 

 ambulacral plates continued to the mouth are, as a group, not of 

 the same taxonomic value, in reference to the Diadematidse, as the 

 other families just mentioned. The Echinothuridae have sub- 

 ordinal characters. On the other hand, it is not possible to 

 separate the Echinidse, Echinometridse, and Temnopleuridse from 

 the Diadematidse ; and they all belong to a suborder. 



There is at present no other method possible than to separate the 

 Grnathostomes which are endocyclic, and which have continuous 

 perignathic girdles and external branchise, into two Suborders. 

 In one the test is flexible and the branchise are both external and 

 internal; and in the other the test is rigid, and the internal 

 branchise are either small, rudimentary, or absent. 



A difficulty occurs amongst the Exocyclic genera, and there is, 

 at present, no satisfactory classification possible of some of the 

 Exocyclica without teeth. 



It appears to be straining a point to separate such a genus 

 as Discoidea from the Order which contains Holectypus, but the 

 perignathic girdle of the last-named and its jaws and teeth 

 diff'er from those of the first-named genus. In Discoidea there is a 

 perignathic collar with possibly the relics of ambulacral pro- 

 cesses ; but it is the opinion of Mr. Percy Sladen and the author 

 of this Revision that no jaws were present. Loven is of a dif- 

 ferent opinion, but still no jaws have been found. It is not 

 possible to place EcMnoconus with either of these genera, for 

 the ambulacra and the peristome diff'er, it has most rudimentary 

 auricles, and the perignathic girdle is reduced to more or less 

 defined interradial ridges, and the genus was jawless. Discoidea 

 was a transition form, between the Grnathostomes and the Suborder 

 of Cassiduloidea, and part of the dental apparatus was abolished, 

 and degeneration of the perignathic girdle occurred. To place 



