REPOKT ON THE ASTEROIDEA. 125 



of the plate. Sometimes one of these spinelets might almost be counted as belonging 

 to the oblique series just mentioned, and sometimes one of them is wanting. 



The occurrence of the pedicellariac on the adambulacral plates in this species is very 

 erratic, some examples being well provided, while others bave very few. The pedi- 

 cellarise, which are large and highly developed, have been figured and described by 

 Perrier. 1 A similar irregularity in the occurrence of the pedicellarise has been noticed 

 by De Loriol 2 in Archaster angulatus ; and I have observed the same fact in specimens 

 from Mauritius in my own collection. 



Family P or cell a n asterid^e, Sladen (1883), emend. 1886. 



Kays usually narrow in relation to the size of the disk, more or less produced. 



Marginal plates in superior and inferior series, thin, lamelliform, apparently naked, or 

 covered only by an extremely thin epidermal tissue. 



Abactinal area covered with membrane, beset with simple spiniferous spicules or 

 pseudo-paxillae, which occupy the whole or only a limited portion of the area. A central 

 epiproctal prominence, more or less defined, frequently developed into an elongate tubular 

 prolongation. 



Actinal interradial areas more or less extensive, paved with thin squamiform inter- 

 mediate or ventral plates, more or less regularly disposed and covered with delicate 

 membrane. 



Adambulacral plates elongate, simple, bearing spines (one to five in number) on the 

 furrow margin only ; or there may be one or more series of small papilliform granules on 

 the actinal surface of the plate. 



Cribriform organs along the vertical sutures of the marginal plates in the interbrachial 

 arcs. 



Ambulacral tube-feet in simple pairs, with conical pointed tips. 



Madreporiform body usually placed close to the marginal plates. 



Remarks. — In this family are included the genera Porcellanaster, Styracaster, Hypha- 

 laster, and Thoracaster, which embrace a series of highly remarkable forms, nearly all of 

 them exclusively from great depths, which were first brought to light during the cruise 

 of the Challenger. I now associate with them the genus Ctenodiscus, whose structure 

 indicates in many ways a community of descent, and shows a much nearer relationship to 

 the present group than to the Astropectinida;, with which it has been previously classed. 

 In many respects, however, Ctenodiscus differs considerably from the other members of 

 the Porcellanasteridse ; and its position in the family must, for the present, be regarded as 



1 Eecherches sur les pedicellaires, &c, p. 95 (separate copy), pi. 2, figs. 12, 13 (Ann. Set. Nat., 1869, 

 t. sii., p. 287). 



2 Mem. Soc. plojs. et hist. not. Geneve, 1885, t. xxix., p. 79. 



