URASTERELLIDvE. 127 



Asterozoa " (93, p. 119). Very generously he leaves me to name this sub-class 

 I do not propose, however, to name any sub-class for the moment, but to await 

 the further completion of my work before attempting a new classification — a task 

 which at present I do not feel competent to undertake. 



The Family Cnemidactinidas is in the fundamental structure of the ambulacral 

 groove the simplest of the group. Unfortunately, however, it is represented solely 

 by one genus and one species, which, like the representatives of Promopalseaster, 

 is found only at its maximum of elaboration just before its final extinction. The 

 want of knowledge of all except this single form renders it desirable to choose the 

 next simplest set of forms, the Urasterellidas, for initial study. 



Family Ueasterelljdj:, Schuchert (emend.). 



1899. T&niasteridte, Gregory (part), Geol. Mag., dec. iv, vol. vi, p. 351 (includes Txniaster, ? Stenaster 



Urasterella, Protasteracanthion, Salteraster). 



1900. Rcemerasterinse, Gregory, Lankester's Treat. Zool., vol. iii, Ecliinoderma, p. 255. 



1914. Urasterellidw, Schuchert (part), Fossilium Catalogus, Animalia, pt. 3, p. 7. 



1915. „ Schuchert (part), Bull. 88, U.S. Nat. Mus., p. 172. 



Diagnosis. — Group C forms with adambulacralia allowing of free movement of 

 the arms. Adambulacral ornament usually specially prominent along a transverse 

 ridge. Infero-marginalia with columnar paxilla-shafts. Apical surface of disc with 

 paired interradialia (proximal adradialia) immediately distal to the primary circlet. 



The family comprises two genera — Urasterella, McCoy, and Salteraster, Stiirtz. 



Schuchert's description of the forms within the Family constituted a great 

 advance on all previous knowledge, for he not only clearly described the American 

 forms but established clear relationships with English, German, Russian, and 

 Australian representatives. His diagnosis is as follows : " Specialised Crypto- 

 zonia, with alternate ambulacra, and with adambulacral type of oral armature. 

 Rays five, rather flexible, long and gently tapering, proximally united without 

 forming interbrachial areas. No inframarginals discernible as such at maturity, 

 the actinal margin being occupied by well-developed adambulacrals. Abactinal 

 area composed of numerous small plates arranged in columns and quincunx. The 

 radial and supra-marginal columns may be discernible in somewhat larger plates. 

 Adambulacrals many, like coins set on edge. Contains : Urasterella, McCoy." 

 Later knowledge compels the radical modification of this diagnosis. 



General Appearance of the Urasterellidse. — A good idea of the general appear- 

 ance of the Urasterellidae may be obtained from a study of the text-figures 

 (p. 129,' p. 139 and p. 167) and the photographs given (Pis. IX, X). The disc is 

 seen to be small, while the arms are often very long. The length of the arm 

 suggests a wriggling form of life, a conclusion borne out by the studies given 

 below. The transition between the various lengths of the arms is shown in the 



