218 



MR. "W. PERCY SLA.DEN ON THE 



to the radius of the disk. Actinal interradial areas paved with thin plates, 

 more or less regularly disposed, but not imbricating and not arranged in 

 definite columns, covered with delicate membrane. Ambulacral furrows 

 wide and exposed. Ambulacral spines one to three in number, elongate, 

 tapering and sharply pointed, usually radiating apart. Cribriform organs 

 one to three in number. Terminal plates large and robust, armed with 

 prominent spines. 



Syiiopsis of Species herein described. 



A. One cribriform organ in each angle, Ambulacral spines 



2-3. No segmental pits or papillas. 

 t Rays short. Dorsal membrane with spinelets confined 



to limited areas cceruleus- 



tt Rays long. Dorsal membrane entirely covered with 

 spinelets. 

 * Rays tapering. Spines on each supero-marginal plate, 

 long and delicate. Terminal plate small, with 5 



spines, Spinelets of disk clavate caulifer. 



** Rays broad and robust. Spines on two plates only, 

 stout. Terminal plate large and tubercular, with 

 3 spines. Spinelets of disk cylindrical tuberosus. 



B, Three cribriform organs in each angle. One ambulacral 



spine. Segmental pits and papillae. 



t Rays long and robust. Dorsal membrane entirely covered 

 with spinelets. Spines on each supero-marginal plate, 

 long and thick crassus. 



tt Rays short and delicate. Dorsal membrane with spinelets 

 confined to limited areas. No spines on the supero- 

 marginal plates or only rudimentary gracilis. 



PORCELLAlSrASTER C^RTJLEUS, WyvUle TllOmsOU. 



Porcellanaster ceruleus, Wyville Thomson {IS"] 'J), Voy. of Challenger,' 

 Atlantic, vol. i. p. 378, figs. 97 & 98. 



Marginal contour substellate, with five moderately developed 



I am not in a position to express an opinion. The smallest examples of that 

 form with which I am acquainted are larger than those named Caidaster 

 fedunculatus by M. Perrier. 



In referring to the alliance of his type with Ctenodiscus, my friend and 

 colleague states that the marginal plates of the young specimens under 

 discussion form only a single row. I venture to think that on closer exami- 

 nation a double row {i. e. supero-marginal and infero-margiual series) will be 

 found to exist, as is distinctly the case in Ctenodiscus. 



The interesting form recently described by Danielssen and Koren under the 

 name of Ilyasfer inirabiUs (Nyt Mag. f. Naturvidensk. 1883, bd. xxviii. 1, p. 4) 

 is imquestionably more nearly allied to the genus Astropecten than any of 

 the forms under notice. 



