20 



BRITISH PALAEOZOIC ASTEROZOA. 



primitive than either the Asteroidea or the Ophiuroidea, and tliat it was from tliis 

 class tliat both the latter classes diverged. My ancestral class contains forms which 

 are devoid of differentiated marginalia (see also p. 8) and possess a marginal 

 madreporite (p. 38). 



The characteristics of the families included in the Recent Asteroidea are (1) 

 the possession of differentiated marginalia at some period of their life-histoz-y 

 (p. 8), and (2) the apical position of the madreporite. It therefore appears 

 desirable to limit the Paleeozoic forms included in the Asteroidea to those which 

 satisfy the above conditions. The advantages which follow from this conrse Avill 

 become obvious when the systematic position of the various groups is considered. 



The Progress to the Asteroidea. 



Certain Asterozoa, e.g. species of Palxasterina, Uranaster, and allied genera, 



PROXIMAL. 



R. Ch. 



Tkxt-fig. 18. — Oral view of ambulacralia and adambulacralia of Uranaster elizsc. Ch., ambiilacral channel; 



R., anibixlaeral ridge ; Ad., adambulacralia. 



appear to be nearly related to very primitive Asteroidea. The ossicles of the arm 

 are arranged simply, and show that adaptations to withstand strain were lacking. 

 A cross-section of the arm of Uranaster elizse, n. sp., is given (Text-fig. 20, p. 23). 

 The ambulacralia form the nsual arch, but supporting marginalia are only slightly 

 developed. 



Text-fig. 18 is based on a study of the aml^ulacral groove when viewed from 

 l}elow. The ambulacralia are seen to be rectangular in plan. Their sides closely 

 touch one another, and there are no pores between them for the passage of 

 ampullge. A well-marked median transverse ridge is present. The longitudinal 

 ridge which forms the apex of the H is but slight, and indicates the feeble 

 development of the transverse ventral muscles. The median margins of the 

 ambulacralia are excavated for a shallow channel, in which lies the radial water- 

 vascular canal. 



Under certain conditions of preservation it is possible to obtain an apical 

 (dorsal) view of the ambulacralia. This view shows even more clearly the absence 

 of ambulacral pores. Excavations for muscle-attachments are either very slight 

 or wanting. The dorsal transverse and longitudinal muscles must have had but 

 slight functional importance in these early forms, and the ambulacralia themselves 



