104 BRITISH PALEOZOIC ASTEROZOA. 



and occupied by single axillary marginal plates. No accessory interbracbial or 

 axillary interbracbial plates present, but tbere are developed accessory abactinal 

 ray ossicles. Actinal plates consisting of ambulacrals, adambulacrals, and infra- 

 marginals. Madreporite abactinal." 



Now, P. niitgareiisis is from tbe Silurian of America and is obviously related to 

 tbe Hudsonasteridse of tliat continent. It has no radialia, but in their place there 

 is a double to a treble row of small polygonal plates. Radialia are such a constant 

 feature in all really primitive forms that we are justified in assuming that these 

 small polygonal plates represent broken-down radialia, and that this may be taken 

 as a sign that P. niagarensis is an end-species of some " Hudsonaster'" lineage. 



Australaster is from a widely different horizon, the Permo-Carboniferous. Only 

 the oral surfaces of the two species are known, and these show features of shape 

 and ornament of the plates widely different from any species from Europe and 

 America. Until we know more about the evolution of these forms from the 

 Southern Hemisphere their exact systematic position must remain doubtful. 



Family Uranastebid^, nova. 



Schuchert (85, p. 138) amends the family Palasterinidee, Gregory, and gives it 

 the following definition : 



" Progressive Phanerozonia developing large interbracbial arcs. Ambulacra! 

 plates more or less alternate. Madreporite abactinal. Disc large, pentagonal, and 

 the rays separated by well-developed interbracbial arcs. The inframarginal plates 

 bound the animal and are separated from the adambulacrals more or less com- 

 pletely by a varying number of interbracbial plates. Abactinal surface with 

 longitudinal columns of radial, supramarginal, and accessory columns of plates, or 

 the radial columns may be reduced or even replaced by transverse rows of small 

 accessory pieces. The central region of the disc may retain a ring of larger basal 

 radials and supramarginal pieces." 



He places in the family the following genera: Petraster, Billings; Limhtro- 

 master, Gregory ; Palasteiiiia, McCoy ; Uranaster, Gregory ; Palseostella, Stiirtz ; 

 PseiLdopalasferina, Stiirtz. 



Many of the forms were only known to Schuchert from illustrations and 

 descriptions, and fundamental redescription is required in some cases. I propose 

 to give new descriptions in the following pages. 



The first genus to be considered is Uranaster, and I propose to place it with 

 forms to be dealt with later in a new family, the Uranaster idge, defining this as 

 follows : 



Asterozoa with well-differentiated infero-marginalia which form both the apical 

 and oral boundaries to the disc and arms. Supero-marginalia also apparently 



