248 ASTEROIDS A AND OPHIUROIDEA. 



or " ossicles/' united together so as to form a species of 

 chain-armour. The ossicles are generally united with one 

 another in a reticulated manner, and the interspaces between 

 them are filled by the coriaceous integument. In some 

 genera there is a single or double row of large plates round 

 the borders of the disc and arms (fig. 140, m). These are 

 termed the " marginal plates." The integument in the Star- 

 fishes is also furnished with spines, tubercles, and granules 

 of calcareous matter. The spines vary in their development 

 and in their position in different Star-fishes ; but there is 

 commonly a row of spines along each side of each of the 

 ambulacral grooves. In some genera (as in Solaster, Luidia, 

 Ctenodiscus, &c.) there are spines the summits of which carry 

 bunches or tufts of minute calcareous processes. These are 

 termed " paxillse." Lastly, in Asteroidea, as in Ecliinoidea, 

 there are the modified pincer-like spines which are known 

 by the name of " pedicellarise." 



As regards their distribution in time, the Asteroidea have 

 a long vertical range, extending from the Cambrian to the 

 present day. In the Upper Cambrian of Britain, remains of 

 members of this order have been detected (Henry Hicks), and 

 others have been described from rocks in Sweden, believed 

 to be of the age of the Lower Cambrian. In the Silurian 

 seas Star-fishes were comparatively abundant, and their re- 

 mains are found, though rarely, in the subsequent Devonian 

 and Carboniferous formations. In the Secondary deposits, 

 and more especially in the Jurassic and Cretaceous, Star- 

 fishes, often belonging to existing genera, are far from un- 

 common ; and other types, closely related to living forms, are 

 found in the Tertiary. 



As regards living forms, the order Asteroidea may be 

 divided into the following five families : 



Family 1. Asteriadce or Asterocanthiidce. Four rows of ambulacral feet. 

 Fam. 2. Astropectinidce. Two rows of ambulacral feet ; back flattish, 



netted with tubercles, which carry radiating spines at the tip 



("paxillre"). 

 Fam. 3. Oreastridce. Two rows of ambulacral feet; skin granular, pierced 



by minute pores. 

 Fam. 4. Asterinidce. Two rows of ambulacral feet ; body discoidal or 



