256 ASTEROIDEA AND OPHIUKOIDEA. 



spines ; but there are never any of those modified spines 

 which are known as " pedicellariae," and which occur in 

 the Asteroids and Echinoids. 



As regards their distribution in time, the Ophiuroids make 

 their first appearance in the Lower Silurian, and they are 

 represented by various ancient types in the Upper Silurian 

 and Carboniferous. The living genus Ophiura is said to 

 occur in the Carboniferous, but with this exception the old 

 forms are all more or less aberrant. In the Secondary 

 rocks, however, we meet with a large number of Ophiuroids 

 which are referable, in large part, to familiar and widely 

 distributed existing genera. 



As the Ophiuroids are, comparatively speaking, very rare 

 as fossils, it is not necessary that we should devote much 

 time to their consideration here. It is advisable, however, 

 to consider with some little detail the curious Palaeozoic 

 genera Protaster, Eugaster, and Ptilonaster, since these ex- 

 hibit many singular and special characters, which are not 

 to be found in the typical and more modern members of the 

 order. These genera, in fact, are in many respects inter- 

 mediate between the Asteroidea and Ophiuroidea ; and they 

 might without impropriety be placed in the Asteroid family 

 of the Brisingidce, were it not for the fact (amongst others) 

 that the under surface of the arms is not furrowed by 

 " ambulacral grooves." 



As the type of the Palaeozoic Ophiuroids in question, we 

 may take the genus Protaster of Edward Forbes. In this 

 genus the body (figs. 147, 148) consists of a circular disc, 

 covered with small imbricated calcareous plates, which gives 

 origin to five long and flexuous arms. The chief peculiarity 

 of the genus is to be found in the structure of the arms, 

 which show the peculiarity that they possess two rows of 

 ventral plates, instead of one, and that these plates are 

 opposite to one another (Salter), or very slightly alternating 

 (Hall). These plates, moreover, are so disposed as to give 

 origin to a series of distinct pores (fig. 148, E). If, as would 

 appear to be the case, Tceniaster of Billings be really iden- 

 tical with Protaster, then the genus dates from the Lower 

 Silurian and ranges into the Devonian. In the Devonian 



