ASTEROIDEA. 251 



ities of their own, more especially the presence of a row of 

 " adambulacral plates," exceeding in size the ordinary plates 

 of the skeleton. They might thus with propriety form a 

 distinct family, which might be termed Palasteriadce, and 

 which would to some extent fill up the interval between the 

 families of the Asteriadce and Astropectinidce. Urasterella 

 may also be best referred to the same group, especially as 

 the plates upon the ventral surface are closely fitted together, 

 though it wants " marginal plates," and in the characters of 

 the upper surface presents points of resemblance to the 

 Urasters. 



We are not, however, without ancient representatives of 

 existing families of Star-fishes. Thus, the Upper Silurian 

 Lepidaster and Trichotaster, and the Devonian Helianthaster 

 represent the living Sun-stars (Solaster), with their wide disc 



Fig. 144. Under surface of Astropecten Phillipsii. of the natural size. 

 Jurassic. (After Wright.) 



and numerous arms ; Devonian and Carboniferous species of 

 the living genus Astropecten (or Asterias) have been detected, 

 and the Upper Silurian Palceocoma of Salter (fig. 143, c) 

 may possibly be an old form of the living " Bird's-foot Star- 

 fishes" (Palmipes). 



Leaving the Palaeozoic period, the Secondary deposits are 

 known to contain a large number of Star-fishes, which need 



