130 BRITISH PALAEOZOIC ASTEROZOA. 



young form of Urasterella ulrichi, in U. asperula (85, p. 174), and at the distal 

 (younger) end of the arm of an adult U. pulchella (85, p. 179). The supero- 

 marginalia, he thought (85, p. 173), might be discernible in the two larger columns 

 of plates which in some species run down the sides of the radialia (X. of Text- 

 fig. 80, p. 128). 



It seems to me that the various series are much more readily recognisable in 

 the English than in the American species. At any rate, I have been always 

 able to distinguish infero-marginalia from the remaining plates in both young 

 and old specimens. Supero-marginalia are never present. The series called 

 supero-marginalia by Schuchert are not present in Salteraster (Text-fig. 94, p. 150) 

 and are clearly the first row of adradialia in Urasterella. 



The fact that there are Asterozoa which possess clearly discernible infero- 

 marginalia but no supero-marginalia is not surprising. Reasons were given above 

 (pp. 76, 77) for the view that even in the case of those Asteroidea which possess a 

 double marginal series, the infero-marginalia precede the supero-marginalia in 



1 ™~ B 



Text-fig. 82.— Drawing illustrating the terms used in describing paxillse. A., paxilla-shaft ; 



B., paxilla-base ; C, paxilla-crown. 



development. The Urasterellidae are one of the groups of forms which separated 

 from the original stock before supero-marginalia were developed, that is, they 

 came from pre-Hudsonaster stock. 



It will be seen that these conclusions are supported by a study of the 

 Cnemidactinidas (p. 155). 



Hudson (93, p. 134) brings forward good evidence that the apical plates fit 

 closely to the tops of the ambnlacralia, and that we must conclude that in the 

 American species the viscera did not extend into the arms beyond the second pair 

 of ambnlacralia. He shows, for example, in the cross-section reproduced 

 (Text-fig. 80) that the first adradial fits into a curve at the top of the right 

 ambulacral. It may be added that the reduction of the dorsal body-cavity seems 

 to have taken place independently in several of the Asterozoan branches. 



Hudson also suggests that the small channels visible between the ambnlacralia 

 and the infero-marginalia (photograph reproduced here, PI. VIII, fig. 7) were for 

 a " fringe of papulas issuing between the infra-marginals and the covering plates " 

 (93. p. 126). 



Structure of the individual Apical Plates (Text-figs. 82, 92). — Schuchert noticed 

 (85, p. 1/3) that a very distinctive feature of the Urasterellidae was the tendency of 



