162 BRITISH PALAEOZOIC ASTEROZOA. 



" During- most of the Palaeozoic, the starfishes could have had no carnivorous 

 enemies other than the cephalopods ; as for marine fishes, the armoured Arthrodires 

 did not appear until the Middle Devonic, while the ancient sharks were not common 

 until Lower Carboniferous (Mississippi) time. It is possible that regeneration 

 among the starfishes is connected with the rise of carnivorous enemies, but as the 

 habit is so common among living forms it is more probable that this power has 

 always been inherent in the class. Regeneration among the crinoids has been noted 

 in several cases where lost distal ends of arms were being replaced by immature 

 growths. Such have been seen in the Lower Carboniferous (Burlington and Keokuk 

 formations) of America." 



Mode of Life. — The rigid position in which the arms have been preserved, and 

 the closely set ambulacralia, suggest that the arm had but little power of lateral 

 or vertical movement. Bather (9a, pp. 451, 510) has already referred to the 

 littoral and probably even arenaceous condition of the " Starfish bed." One of 

 the commonest of recent Starfishes in a littoral arenaceous habitat is Astropecten, 

 which has pointed feet by means of which it runs about the sandy bottom. It 

 seems to me that Onemidactis may have had a similar mode of life. 



The condition of the mouth-parts suggests also that the creature fed just as 

 does Astropecten. The rigid form of the mouth-rings and the trap-like nature of 

 the tori of Onemidactis are all in favour of this view. MacBride (43, p. 468) 

 states that the " loss of suckers has rendered Astropecten and its allies incapable of 

 feeding in the manner described in the case of Asterias rubens. They are unable 

 forcibly to open the valves of shell-fish, and the only resource left to them is to 

 swallow their prey whole. The mouth is consequently wide, and the unfortunate 

 victims, once inside the stomach, are compelled by suffocation to open sooner or 

 later, when they are digested." 



Family ArthrasteriDjE, nova. 



Diagnosis. — Group C forms with adambulacralia much as in the Urasterellidae. 

 Infero-marginalia with paxilla-shaft in the form of a strong transverse ridge. 

 " Single" interradial plates immediately distal to the primary circlet. 



No previous attempt has been made to link up the forms described below with 

 other Palaeozoic Asterozoa. I have named the family after the Cretaceous genus 

 Arthraster, which seems to have been the longest surviving genus exhibiting " Uras- 

 terellid " affinities. The forms in the family have the long arms, small disc, and 

 large adambulacralia of the Urasterellidse, but differ in the apical plates, which 

 usually have paxilla-shafts in the form of a transverse ridge. Superficially similar 

 Asteroidea with long arms and a small disc, such as Odinia and Labidiaster, also 

 occur in the existing deep sea ; but they have evolved along quite different lines 

 and are probably of more recent origin. 



