THE STELLEROIDEA 251 



their free ends, and the rays taper gradually (Taeniaster). Genera 

 Tacniaster, Billings ; Stenaster, Billings, pars (S. salleri, but not S. pul- 

 chellus) ; both Ordovician of Canada ; Salteraster and Urasterella, M'Coy, 

 Silurian, England ; Protastemcanthion, Stu'rtz, Devonian, Germany. 



FAMILY 5. ARCHASTERIDAE. Phanerozonia with opposite ambulacral 

 plates. There is an anus, but no super-ambulacra! plates. Pedicellariae 

 are generally present. The abactinal plates are spiniform or paxilliform. 

 The adambulacral plates are large. This family includes a large number 

 of Neozoic starfish, ranging from the Lower Oolites to the present day. 

 They have frequently been included with the Astropectinidae, from 

 which they differ by the presence of an anus, by the large size of the 

 adambulacral ossicles, and by the absence of supra-ambulacral plates. 

 There are four sub-families, including sixteen genera. SOB-FAMILY 1. 

 PARACHASTERINAE, comprising those in which the branchial vesicles or 

 papulae are limited to an area at the base of the rays, and in which 

 the actinal interradial plates are absent or are very few in number. 

 Genera Cheiraster, Studer ; Pararchaster, Sladen ; Pectinaster, Perrier ; 

 and Pontaster, Sladen. SUB - FAMILY 2. PLUTONASTERINAE, including 

 those with papulae scattered over the whole abactinal surface. There 

 are numerous actinal interradial plates. Genera Crenaster, Per. (non 

 Ag.) ; Dytaster, Slad. ; Goniopecten, Per. ; Lonchotaster, Slad. ; Persephon- 

 aster, Mason & Alcock ; Plutonaster, Slad.; Tethyaster, Slad. SUB- 

 FAMILY 3. PSEUDARCHASTERINAE, including those with a definite median 

 line of plates along the rays, and with the abactinal plates arranged 

 in series parallel to the central line. There are no pedicellariae. 

 Genera Pseudar 'chaster, Slad. ; Aphroditaster, Slad. SUB-FAMILY 4. ARCH- 

 ASTERINAE, including those in which there is a definite median line of 

 abactinal plates, and the remainder are arranged in oblique rows. 

 Pedicellariae present. Genera Acantharcluister, Verrill ; Archaster, Verr. ; 

 Isaster, Verr. The following genera are included by Sladen in this 

 family, but not divided among the sub- families : Benthopecten, Verr.; 

 Blakiaster, Per. ; Luidiaster, Stud. 



FAMILY 6. PORCELLANASTERIDAE. Phanerozonia with opposite ambu- 

 lacral plates ; with thin lamelliform marginal plates, traversed by cribri- 

 form organs. There are two sub-families. SUB-FAMILY 1. PORCELL- 

 ANASTERINAE, in which the cribriform organs are highly developed and 

 limited to a few plates, and there are no fimbriated channels in the 

 actinal interradial areas. Genera Caulaster, Per. ; Porcellanastei; Wyv. 

 Thomson (Fig. VIII.); Styracaster, Hyphalaster, Thoracaster, and Pseiid- 

 aster, Sladen. SUB-FAMILY 2. CTENODISCINAE, in which the cribriform 

 organs are simple and occur on the ma'rgins of each pair of marginal 

 plates ; continuations from the cribriform organs run through the actinal 

 interradial areas as " fimbriated channels." Genus Ctenodiscus, Miiller & 

 Troschel. The most interesting features of this family are the develop- 

 ment of the cribriform organs, densely packed groups of small spinelets or 

 lamellae on some or all of the marginal plates. Their function is uncer- 

 tain, but, according to Sladen, " it is not improbable they act as percolators." 

 In some species of Astropecten the marginal ossicles are bordered by 

 fringes of small spines, which A. Agassiz has compared to the fascicles of 



