OPHIUROIDEA. 



255 



skeleton. The internal skeleton consists, as in the Star- 

 fishes, of a chain of " ambulacral ossicles," placed along the 

 centre of the arm. The ossicles are, however, now amalga- 

 mated in pairs, each coalescing with its fellow on the oppo- 

 site side ; so that in place of a double row of movably 

 articulated ossicles, we have a single row of bilaterally 

 symmetrical pieces. 



Fig. 146. Under surface of Ophioderma (Ophioglypha ?) Gaveyi, of the natural size. 

 Jurassic (Lias). (After Wright.) 



The external skeleton of the arms is composed of four 

 rows of plates, one on the dorsal surface, one on the ventral, 

 and one on each lateral surface. The lateral plates generally 

 carry more or less developed spines. In the extinct genera 

 Eugaster and Protaster (fig. 148) the plates of the ventral 

 row are double; and in Ptilonaster (fig. 148) there are four 

 ventral rows of plates. The disc, as before said, has a 

 well - developed external skeleton of scales, granules, and 



