OPHIUROIDEA. 



405 



ary, and as corresponding with the ventral shields (" superambula- 

 cral plates ") of the normal Ophiuroids, their special peculiarity on 

 this theory of their nature being their duplication. On the other 

 hand, it may be held that the ventral or superambulacral plates of 

 the ordinary Brittle-Stars are not developed at all in the ancient 

 types in question, and that the double row of plates above alluded 

 to are really the " ambulacral ossicles " or " vertebral discs," which 

 would, on this view, differ from the corresponding structures in the 

 normal Ophiuroids by being separate, instead of being anchylosed 

 in pairs. If this latter view be correct, the Protophiurids may have 

 either had open ambulacral grooves, as in the Asteroids, or the 

 under side of the arms may have been closed by soft skin, as in the 

 Euryalids. Whichever of these two views is the correct one, the 

 plates of the double ventral row of the Protophiurids may either be 

 placed opposite one another, or they may alternate, and they are 

 also so disposed as to give origin to a double series of pores. 



Of the genera of Protophiurids, Protaster (fig. 282) is found in 

 the Ordovician and Silurian rocks, and is characterised by its round 

 scaly disc, and by the fact 

 that the arms are provided 

 inferiorly with a double row 

 of plates, and carry bunches 

 of lateral spines (fig. 282, b). 

 In the Devonian genus Eu- 

 gaster (fig. 281, a) the gen- 

 eral structure is very similar 

 to that of Protaster, but the 

 disc is prolonged along the 

 bases of the arms, and the 

 plates of the disc are articu- 

 lated by their edges, and do 

 not overlap. The under side 

 of the arms (fig. 281, b) ex- 

 hibits a double row of alter- 

 nating plates, and lateral 

 spines appear to have been 

 wanting. In the genus 

 Pti/onaster, again, the arms 

 exhibit on their under surface 

 four rows of plates and a 

 double row of pores. This 

 £enus is also Devonian. 



282. — Protaster Sedgwickii. Silurian, a, 

 Disc and bases of the arms, magnified ; B, Portion 

 of an arm, greatly enlarged. (After Salter.) 



Lastly, the genus Tceniaster, 



from the Ordovician rocks of Canada, is in many respects like 



Protaster, but the two rows of plates which occupy the under sur- 



