128 BRITISH PALEOZOIC ASTEROZOA. 



Plate. A cross-section through the arm shows even more clearly some of the 

 essential differences between these forms and the Asteroidea of Section A, espe- 

 cially if the two sections be placed alongside as they are in Text-figs. 70, 80. It 

 will be noticed that in Promopalxaster the adambulacralia are comparatively small, 

 and the great mass of the calcareous skeleton consists of the marginalia and the 

 apical plates, while in Urasterella the adambulacralia are remarkably stout and it 

 is difficult to distinguish marginalia. Further the calcareous skeleton of Promo- 

 pal&aster encloses a large body-cavity which helps to contain the viscera, gonads, 

 etc. In Urasterella as we shall see (below, p. 130) there is distinct evidence 

 that the viscera scarcely entered the body-cavity. Another important feature of 

 the Urasterellidae is the paxillar character of the apical and marginal plates. All 

 these features are dealt with in detail below. 



79 80 



0-S.M. 



<>Q 



IM. 

 Ad. 



Text-fig. 79. — Cross-section through an arm of Promopalxuster eliza (slightly diagrammatised). Ad., 

 adambulacral ; Am., amljulacral ; I.M., infero-marginal ; E., radial ; S.M., supero-marginal. x ('•. 



Text-fiu. 80. — Cross-section of the arm of Urasltrella yvlchrlla (after Hudson). Ad., adambulacral; 

 Am., ambulacral ; I.M., infero marginal ; R., radial; X., first adradial. Notice the first adradial 

 on the right side fitting into a concave depression on the ambulacral. x 15. 



Arrangement of the Apical Plates (Text-fig. 81; Text-fig. 88, p. 138; Text- 

 fig. 92, p. 143). — A type of arrangement of the apical plates which may un- 

 doubtedly be regarded as primitive, is well shown in four English specimens 

 (one adult Urasterella thraivensis, n. sp. ; one young of same species; two adult 

 U. ruthveni, var. leinticardiiiensis) and one American species, a young form of 

 U. ulrichi, Schuchert (85, pi. 30, fig. 6). The centre of the disc is occupied by a 

 centrale which may exceptionally appear as a double ossicle. External to this is 

 a circle of six plates, five of which are in series with the radialia and are 

 undoubtedly primary radialia, whilst the sixth is in the madreporic interradius 

 just proximal to the madreporite and is a single primary interradial. It is clear 

 that this arrangement is more primitive than that in the most primitive Asteroidea 

 of Section A (the Hudsonasteridae), for these have a centrale separated from the 

 primary radialia by a ring of accessory plates and never possess less than five 

 primary interradialia (see above, p. 61). Schuchert, in spite of his frequent 



