ARTHRASTERID^. 



163 



There are at present only three genera belonging to the family, Protarthr aster 

 from the English Devonian, Galliasterella from the Russian Carboniferous, and 

 Arthraster from the North-west European Cretaceous. The distinctions of the 

 forms from one another and from the Urasterellidas can best be seen from the 

 following table : 





Urasterellidre. 



F rot arthraster. 



Calliasterella. 



Arthraster. 



Apical. 



Marginal. 



Apical. 



Marginal. 



Infero- 



Paxilla-shafts usually 



Paxilla-shafts as 



Paxilla-shafts 



Paxilla-shafts 



marginalia 



columnar, rarely as 



transverse ridges 



as transverse 



as transverse 





transverse ridges 





ridges 



ridges. 



Radialia 



Paxilla-shafts either 



Paxilla-shafts as 



Paxilla-shafts 



Paxilla-shafts 





ridge-like, boss -like, 



transverse ridges 



as transverse 



as transverse 





or columnar. If there 





ridges 



ridges. 





are ridges they are 











not transverse, but 











run the length of the 











ossicles 









Adradialia . 



First row may have 



First two rows with 



Not present 



One row with 





longitudinal ridges or 



transverse ridges, re- 





transverse 





all may have long 



mainder with short 





ridges. 





columnar paxilla- 



columnar paxilla- 









shiifts 



shafts 









Proximal adradialia 



Proximal adradialia 



Proximal adradi- 



Disposition of 





paired 



single 



alia single 



proximal adradi- 

 alia not known. 



The arrangement of the apical plates of the disc gives a clue to the ancestry of 

 the forms within the Family. 



Schondorf regarded the " single " interradial plates of Calliasterella (Text- 

 fig. 110, p. 1G7) as primary interradialia homologous to the similar plates of the 

 Asteroidea of Section A. This cannot be, as the primary interradialia are always 

 proximal to the primary radialia, whereas in this form the plates are distinctly 

 distal. It seems much more reasonable to suppose that the plates have arisen 

 from a suppression of one of each of the paired proximal adradialia of the Uras- 

 terellidas, as is foreshadowed in S 'alt eras ter ? coronella (Text-fig. 90, p. 140), and 

 that a circlet of primary interradialia is not present in Calliasterella just as is 

 the case in Urasterella. The presence of the " single " interradial in Protar- 

 thraster is an important link between Calliasterella and the early " Urasterellid " 

 forms. It is possible that when a disc of Arthraster can be examined it will 

 have the same structure, for this genus obviously has close relationships with 

 Calliasterella. 



I have divided the family into three sub-families corresponding to the different 

 genera. It may be that when further connecting links are known this sub-division 

 may prove to be unnecessary. 



