OPHIUROIDEA. 117 



As regards their distribution in time, the Ophiuroidea are 

 represented for the first time in the Upper Silurian Rocks by 

 the genus Protaster (fig. 70), unless Tceniaster be rightly re- 



Fig. 70. Protaster Sedgwickii. Upper Silurian. A, Disc and bases of the arms, 

 magnified. B, Portion of an arm greatly enlarged. 



ferred here, in which case the order begins in the Lower 

 Silurian. No other Palaeozoic genera have been satisfactorily 

 made out. In the Muschelkalk, the middle member of the 

 Trias, we have a well-marked species, the Aspidura loricata of 

 Goldfuss (fig. 71). In the Oolitic, Cretaceous, and Tertiary 



Fig. 71. Aspidura loricata. Muschelkalk. 



Rocks, Ophiuroids are by no means uncommon, and they be- 

 long for the most part to genera which exist at the present day. 

 Great uncertainty prevails as to the generic characters of these 

 animals ; but the Mesozoic forms are for the most part gene- 

 rally referred to the genera Ophiolepis, Ophioderma, Ophiocoma, 

 Amphiura, and Acroura. 



