168 BRITISH PALEOZOIC ASTEROZOA. 



and Worthen. I have already noted (p. 150) a somewhat similar method of 

 preservation of the English species Salteraster asperrinnis and the American 

 species Urasterella grandis. It is probably due to a strong contraction of the 

 interadambulacral muscles after death, and indicates the powerful character of 

 these muscles (see p. 133). 



Text-fig. 110 is a plan of the ossicles on the apical surface of the disc re- 

 lettered so as to show its correspondence with a similar plan of the same region in 

 Protarthraster (Text-fig. 104, p. 164). The prominent odontophor is much more 

 exposed than in Protarthraster, and reminds one of the similar ossicle in Cnemi- 

 dactis (compare with Text-fig. 101, p. 157). 



Text-fig. Ill shows " Urasterellid " adambulacralia with long spines. Schon- 

 dorf (op. cit., p. 332) describes the proximal (adoral) surface of the adambulacralia 

 as being concave and the distal (aboral) surface as being convex. I am inclined to 

 believe after examination of Schondorf's figures that the muscle-attachments are 

 merely a modification of those described for Urasterella thraivensis (p. 133). Xo 

 adambulacral ridges have been described. The adambulacral armature is, how- 

 ever, very similar to that of Urasterellx (compare PI. VIII, fig. 3). It is just 

 possible that very reduced adambulacral ridges may be present, as in Urasterella 

 montana (p. 148). The body-cavity as depicted by Schondorf is much larger than 

 in any of the Urasterellidse. 



Text-fig. 112 shows the general resemblance between the mouth-parts of the 

 form and those of Urasterella. The Text-figure should be compared with fig. 4 of 

 PI. VIII. 



Sub-family Arthrasterinse, nova. 



1907. Arthraster (family uncertain), Spencer, Mon. British Foss. Echinoderm., Cretaceous 



(Palseontogr. Soc), vol. ii, p. 91. 

 1913. „ „ Spencer, Phil. Trans. Roy. Soc, vol. ccivB, p. 139. 



Diagnosis. — Arthrasteridas with five rows of apical plates (one row of radialia, 

 two rows of adradialia, and two rows of infero-marginalia). These rows have 

 paxilla-shafts in the form of strong transverse ridges. Arrangement of the apical 

 plates of the disc and the mouth-parts not known. 



The Sub-Family contains one genus, Arthraster, with two species, A. <li,c<>iti, 

 Forbes, and A. crista t us, Spencer. Both species are found in the Chalk of X.W. 

 Europe. I am dealing briefly with this genus here although it has no known 

 Palaeozoic representative. It is convenient, however, to place the forms at 

 least in relationship with other Asterozoa. 



