INTRODUCTION. 51 



There remain accordiiioly only two classes of Palaeozoic Asterozoa — the 

 " Asteroiclea " and the " Anluroidea " — il' they are divided as is suggested by 

 Schondorf. Tiie latter correspond to my primitive " wrigglers." The classifica- 

 tion, then approximates closely to that Suggested by Professor and Miss Sollas. 



These authors (71, 72) have made very exhaustive investigations of Laptrortltura 

 miltoni and Oni/rJtasfer fie,vilis, and have drawn comparisons between the structure 

 of these forms and that of EopJiinra, Pah%tira, and Bohemura (as described by 

 Jaekel), Pi-dtaster sed(jwicl-l, VrQiader (jroonii, Rluxlodoma {Protaster) Ippfm^oma, 

 Farcai^fer, Eolii'uVKi, Eospondylus, and Miospondyluft. All these forms are 

 regarded by the authors as being Silurian and Devonian Ophiuroidea, and are 

 placed in a group, " the Frotophmroidpn,'^ of whicli the diagnostic characters 

 are (1) the comi)leteness of the ambulacral series (the first pair of ambulacral 

 ossicles [<7j] l)eing retained), (2) the meeting of the ambulacral series of 

 adjacent arms on the actinal sides of the jaws, (o) the absence of npper and 

 under arm-plates. 



All the modern genera form a group, " the Enophinroidca,'" characterised 

 (1) by the specialised buccal armature, consisting of oral angle pieces formed by 

 the complete fusion of a.2 and ad^, a^ having been lost; (2) by the presence of 

 under arm-plates, with the single exception of Ophioteresis. 



They state that in respect to the three characters given, " the structure of the 

 Protophiuroidea is Asteroid in nature. The actinal position of the madreporite is 

 also shared by some of the early Asteroidea, so that the sharp limitation of the 

 arras from the disc is the chief distinguishing character between the early Star- 

 fishes and Brittle-stars." They also believe that " within the Protophiuroidea a 

 gradual evolution can be traced. In the most primitive genera the ambulacral 

 ossicles are neither completely alternate nor completely opposite. As examples of 

 this most primitive structure Eophiura, Falseiira, and Bohemura may be quoted. 

 They are further characterised by the large number of ambulacral ossicles which 

 bound the oral angles and by the absence of fusion with one another in a 

 longitudinal series of these ossicles. For the forms with alternating ambulacral 

 ossicles Grregory proposed the order Lysophiurse. We were at first inclined to 

 retain it as a division of the Protophiuroidea, referring the remaining members of 

 the sub-order to a division Synophiurse characterised by opposite ambulacral 

 ossicles. But the state of knowledge at the present time hardly permits of this. 

 For while it is certainly clear from Jaekel's work (see, for instance, his figs. 1 and 

 6) that in some of the simplest genera the ambulacral ossicles were not opposite, 

 it is also certain that free opposite ambulacral ossicles are easily displaced, either 

 during life or after death, and it is difficult in some cases to be sure whether 

 alternation is natural or due to displacement. We therefore think it wisest for the 

 time merely to recognise the fact that there has been a progression within the 

 Protophiuroidea from forms with free, partly alternating, ambulacral ossicles, on 



