PAXILLOSA 



467 



Spinulosa, such as Echinasteridae, and the other families of the 

 Paxillosa some genera, indeed, might almost be classed as 

 Spinulosa. At the same time they are apparently closely allied 

 with the more primitive Valvata such as Astrogonium and its allies, 

 some of which have paxillae on the upper surface ; although the 

 retention of the anus and of the suckers on the tube-feet (in 



Fig. 200. Aboral view of Archaster bifrons, 



(From Wyville Thomson.) 



which characters they agree with the Archasteridae) distinguishes 

 them from the more typical Paxillosa, in which both anus and 

 suckers are lost. Archaster (Figs. 200, 201). Leptogonaster. 



Fam. 2. Astropectinidae. Paxillosa which have lost the 

 anus, but which possess neither aboral protuberance nor inter- 

 radial grooves. The marginal plates are thick, covered with 

 spinules and placed horizontally. The tube-feet have no suckers. 



This family is the only one of the order which occurs in 

 British waters, where it is represented by two genera, Astropecten 

 and Luidia. In Astropecten the inferior marginal plate is in 



