126 



THE VOYAGE OF H.M.S. CHALLENGER. 



an isolated one. The structural characters of this genus appear to me to warrant its 

 separation in a distinct subfamily. 



The Porcellanasteridse may be said to hold a divergent position between the Archas- 

 teridiB and Astropectinida?, rather than a directly intermediate relation. In some respects 

 they appear to present a more archaic character than the Archasteridas, but this, it seems 

 to me, is dependent on striking features of structural detail rather than on general facies, 

 and, as a matter of fact, is not borne out by the present state of our knowledge of the 

 older forms of starfish life. Furthermore, this apparently archaic facies is largely due to 

 the remarkable degree in which the Porcellanasteridse present permanently characters 

 which are regarded, in the majority of forms at least, as essentially embryonic. 



The following table will show the distinctive relations of the genera discussed in the 

 succeeding pages, and will serve as a brief epitome of the more prominent superficial 

 characters by which they may be recognised. The genera are divided into two subfamilies. 



Synopsis of the Genera included in the Family Porcellanasterid^:. 



A. Cribriform organs highly developed; localised. Actinal interradial areas 

 with no fimbriated transverse channels ...... 



a. Actinal intermediate plates naked. Adambulacral plates with a single 



series of spines on the furrow margin only. Terminal plates 

 large, armed with conspicuous spines. Cribriform organs one 

 to nine in number. 



a. "With a tubular epiproctal elongation or anal funnel. Abactinal 



membrane with simple spiniferous spicules. Actinal inter- 

 mediate plates not imbricated. Ambulacral furrows wide 

 and exposed. Rays more or less turned back. Cribriform 

 organs one to three, component structure lamelliform . 



b. No tubular epiproctal prolongation, but a more or less definite 



conical peak may be present. Abactinal membrane with 

 pseudo-paxillse. Actinal intermediate plates imbricated and 

 arranged in columns. Ambulacral furrows narrow and con- 

 cealed. Rays not revertible. Cribriform organs five to nine 

 (three 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. Spines of adambulacral armature 

 long and needle-shaped, radiating apart. Mar- 

 ginal plates united along the median line of the 



rays 



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

 Spines of adambulacral armature short, com- 

 pressed, forming independent series or fans 



b. Actinal intermediate plates covered with spiniform granules. Adam- 



bulacral plates with papilliform spinelets on the outer portion 

 of the plate. Terminal plates very small and inconspicuous ; 

 unarmed. Cribriform organs fourteen in number . 



PORCELLANASTERIN^. 



Porcellanaster. 



Stijracaster. 



Hyplialaster. 



Thoracaster. 



