EOACTINTD^E. 



181 



growth takes place so that the madreporite (one of the month -shields perforated 

 for the purpose) is forced round on to the oral side, the plate still retaining its 

 association with the second adarabulacral. As Sollas has pointed out, this is the 

 position of the madreporite in the Silurian Ophiuroid LapwortJiura, and I hope to 

 show that the position is so universal in the Ophiuroidea that we can suppose the 

 impulse to have arisen at the same developmental period and in the same way in 

 almost all, if not all, Ophiuroidea. 



Palseontological evidence supports the view deduced from embryology. There 

 can be no doubt that the Ophiuroidea separated from the parent Asterozoan 

 stock in very early times. We find readily recognisable Ophiuroidea with well- 



Text-fig. 123. — A stage in the development of the young AirvpMv.ro, aquamatn (after Ludwig). Ad. t , Ad. x , Ad. 4 , 

 the second, third, and fourth adambulacralia ; 31., the mouth-shield, which functions as a madreporite. x 140. 



developed vertebras and large discs in the Ordovician rocks — that is, more or 

 less contemporaneously with the oldest known fossil Asteroidea. We also find 

 in these rocks one or more series of forms which have not undergone such rapid 

 evolution and show the early fossil stages of the evolution of the Ophiuroid disc. 

 Such a form is Schuchertia, which is known from the Middle Ordovician to the 

 Middle Silurian. If Ave look at the oral surface of this (see below) we see that the 

 infero-marginalia and odontophor are arranged on the same common ground-plan 

 as in Hudsonaster and Urasterella. The arms, however, could never fuse at the 

 bases because they were separated by a number of interradial plates. Further, 

 the oral position of the madreporite and its association with the proximal adam- 

 bulacralia suggest most strongly that these interradial plates have arisen, as in 



