ASTEEOIDEA OF H.M.S. 'CHALLENGER' EXPEDITION. 215 



Synopsis of Genera included therein. 



A. Ventral plates naked. Adambulacral plates 



with a single series of spines on furrow-margin 



only. Terminal plates large, and armed with 



conspicuous spines. Cribriform organs 1-9 



in number. 

 a. Tubular epiproctal elongation. Dorsal mem- 

 brane wdth simple spiniform spicules. 

 Ventral plates not imbricated. Ambu- 

 lacral furrows wide and exposed. Rays 

 more or less turned back. Cribriform 

 organs 1-3 : component structure lamel- 



liform PORCELLANASTER. 



h. No tubular epiproctal prolongation. Dorsal 



membrane with pseudo-paxillae. Ventral 



plates imbricated and arranged in columns. 



Ambulacral furrows narrow and concealed. 



Rays not revertible. Cribriform organs 



6-9 (3 in one case only) : component 



structure papilliform. 

 a. Rays very long. Supero-marginal plates 

 with long, robust spines, forming a single 

 series along the median line. Ambu- 

 lacral spines long and needle-shaped, 

 radiating apart. Marginal plates united 



along the median line of the ray Styracastee, 



/3. Rays short. No spines on the supero- 

 marginal plates. Ambulacral spines 

 short, compressed, forming independent 

 series or fans Hyphalaster. 



B. Ventral plates covered with spiniform granules. 



Adambulacral plates with papilliform spinelets 

 on the outer portion of the plate. Terminal 

 plates very small and inconspicuous, unarmed. 

 Cribriform organs 14 in number Thoracaster. 



Note on the " Cribriform Organs.'" — A peculiar structure, 

 apparently associated with special functions, occurs in this 

 group ; and, in the form here described, is as yet unknown in 

 other Starfishes. The structure in question is situated on the 

 marginal plates in the interbrachial angle ; and the number of 

 the supposed organs, which is constant in a species, may vary 

 from one to seven or even more in each angle. The following 

 brief account will indicate the general character of the organ 

 throughout the series. In P or cellan aster the marginal plates 

 are of uniform thickness, and form a level plating, the successive 

 plates fitting close together and are not separated by any vertical 

 furrow or marginal bevelling of the plate. In a species possess- 

 ing only one of these organs in each angle, the structure about to 



