50 



ASTEROIDEA, 



Third Family. 



Two rows of ambulacral suckers in the avenues ; without a vent. 



16. Genus. — Astropecten, Linck.= Stellaria, Nardo; Asterias, Agass. 



Body stellate, flat on both sides, with elongate rays. Two rows of large marginal 

 Fig. 28. plates at the border. The lower series 



provided with spine-like scales, which 

 increase from within outwards into long 

 moveable spines. The dorsal marginal 

 plates are covered with granules, which 

 often become spinous, and sometimes 

 carry spines. The flat upper surface of 

 the body and rays thickly covered with 

 appendages, the summits of which are 

 crowned with groups of minute spines, as in Archaster. 



Portion of a ray of Astropecten polyacanthus, M. and T. 

 surface ; B, the upper surface of the ray. 



A, under 



17. Genifs. — Ctenodiscus, M. and T. 



Body flat, almost pentagonal ; at the border two rows of marginal plates, which are 

 completely smooth on the upper surface. The marginal plates on the under surface form 

 transverse bands, which are clothed with scales. A comb-like ridge of fine spines 

 surrounds the border of these plates. The upper marginal plates have a row of large 

 spines at the border, and the basal marginal plates support a row of similar spines near 

 the avenues. The dorsal surface is covered with paxillae, as in Astropecten. 



18. Genus. — Luidia, Forbes. 



Fig. 29. 



A. B. 



Portion of a ray of Luidia Senegalensis. M. and T. A, the upper ; 

 D, the under surface. 



Rays elongate. Instead 

 of a double row of marginal 

 plates, only a single row on 

 the under side, with spines. 

 The entire of the dorsal 

 surface covered with pax- 

 illae, as in Astropecten and 

 Ctenodiscm. 



