204 



BRITISH PALEOZOIC ASTEROZOA. 



forms are transitional between the early Asterozoa and the true Ophiuroidea, it 

 must not be assumed that any of their descendants became typical Ophiuroidea. 

 Those Eoactinid genera which we know adequately are specialised forms, which go 

 through their own lineage-development and die, without leaving, so far as we can 

 judge, any descendants. 



.M. 



Tkxt-fiq. 145. — Diagrammatic cross-section through the arm of Schuchertia wenlocki. Ad., adambulacral ; 



Am., ambulacral; I.M., infero-marginal. x 10. 



Much material is known of Eoactinidas. Nearly the whole of this divides into 

 two lineages, each of which can be confined to one genus. These two genera are: 



Schuchertia, Gregory, found both in Britain and America, and ranging from the 

 Middle Ordovician to the Middle Silurian. 



R. 



Mr" 



S.M. 



A 



m. 



Text-fig. 14-6.— Cross-section through the arm of I'alasterina primxva. Ad., adambulacral ; 

 Am., ambulacral j R., radial : S.M., supero-marginal. x 10. 



Palasterina, McCoy, found in Britain, Sweden and Germany, and ranging from 

 the Middle Silurian to the Lower Devonian. 



A distinctive feature between the two lineages is the shape of the mouth-angle 

 plates, figured in Text-figs. 143 and 144. 



In addition, as mentioned above, Schuchertia has a flatfish arm and well-differen- 

 tiated infero-marginalia, while in Palasterina the arms are much more rounded. 



