:]() BRITISH PALAEOZOIC ASTEROZOA. 



(b) The F(jslt'u)ii nf fJie Madrt'poiife. —Kecewt work lias done iniicli to clear up 

 the various problems connected with the position of the madreporite in Palaeozoic 

 forms. Answers have to be found to the following questions : 



(i) Which of the positions, oral, apical, or marginal, is the priuiitive one? 

 (ii) At what stage did the migration of the madreporite from one surface to 

 the other occur ? 



Various misconceptions have arisen in the discussion of these two points. A 

 clear account of these lias recently been given by Sollas and Sollas (71), and the 

 (juotations given below are from their paper. 



Stiirtz at various times described a number of fossil Asterozoa regarded by him 

 as belonging to the Asteroidea which had an actinal (ventral) madreporite. His 

 observations led him to suggest (78, 181)0) " that if the madreporite originally 

 occurred on the same aspect of the body in both Starfishes and Brittle stars, 

 it was actinal in both, and that it is in the Starfish and not in the Brittle 

 star that a change has taken place." He called attention to a prediction made 

 by Sladen in 1880, who, commenting on Agassiz's statement (1) that in very 

 young starfish the madreporite is at first ventral (actinal) and only subsequently 

 ])ecomes dorsal (abactinal), suggested that ancient fossil starfish would be found in 

 which the madreporite retained its ventral (actinal) position in the fully adult 

 state. Stiirtz added that this prophetic remark had not long to wait for justifica- 

 tion ; several fossil starfish were already known at that date in which the madre- 

 porite lies on the actinal surface of the adult. 



In 1890 the situation was obscured by the observations of Gregory (26), who, 

 in his description of Lapivorthura miltoni, followed Salter (see p. 40) in assigning 

 the madreporite to the dorsal surface. 



Salter had in consequence of this observation transferred the species to the 

 Asteroidea. Gregory retained it in the Ophiuroidea, to which it had rightly been 

 transferred by Stiirtz, and definitely stated that the madreporite was dorsal in 

 some Ophiuroidea. 



This led Stiirtz (80) to draw " attention to the difficuUy which is introduced 

 into the phylogeny of Stelleroidea by Gregory's description of Lapicorthura and of 

 Encladia, and his statements that Bury had found a doi'sal madreporite in young 

 Ophiuroidea; for, he argued, while in Starfish we have modern forms with a 

 dorsal (abactinal) madreporite and primitive forms with ventral (actinal) madre- 

 porite, and according to Agassiz a veutral (actinal) madreporite in young star- 

 fish, in the Ophiuroidea, on the other hand, the converse appears to be the case. 

 Again, if in the original stock of Ophiuroidea the madreporite lay on the abactinal, 

 and in the original stock of the Asteroidea on the actinal side, this fact would not 

 lead to the view that the Ophiuroidea and Asteroidea were derived from the 

 s;ime stock." 



Prof, and Miss Sollas and Schondorf, working independently, have been able to 



