156 BRITISH PALAEOZOIC ASTEROZOA. 



the form to show features more like the primitive Asterozoa than do any con- 

 temporary relations which have evolved from the parent stock at a later date than 

 it did. It is these ancestral characters which give much interest to the study of 

 the form. These points can clearly be seen from the following table. 



(1) Features which shoiv the Relationships with the Urasterellidse. 



(a) The small disc and long arms. 



(b) The structure of the cross-section of the arm (Text-fig. 100). The 



adambulacralia occupy the whole of the oral surface and a portion 



of the margin. There is a row of clearly differentiated infero- 



marginalia above these. 

 (r) The apical covering which consists of ossicles each with a long process 



very like the paxilla-shaft of the Urasterellidse. 

 {d) The structure of the mouth-parts which are typically Urasterellid. 



That the form is related to the Urasterellidse was seen by Nicholson 



and Etheridge when they first described it (see below) and by 



Schuchert. 



(2) Features which are more Primitive than those of the Urasterellidse. 



(1) The adambulacralia are covered by a large number of small spines. 



In Urasterella specialisation has resulted in the restriction of the 

 spines to a ridge, and the spines themselves are very long and 

 stout. 



(2) The nose of the adambulacralia is not so much modified as in the 



Urasterellidse. 



(3) The infero-marginalia are more similar to those usually found in the 



Asteroidea, and have a covering of small spines. In the Urasterellida? 

 these marginalia have become modified into paxillse. 



(4) There is a large torus. 



Genus CNEMIDACTIS, novum. 1 



1. Cnemidactis girvanensis (Schuchert). Plate VIII, figs. 5, 6; Plate XI, figs. 

 1, 2; Plate XII, figs. 1-5; Plate XIII, figs. 1, 2; Text-figs. 100—103. 



1880. Tetraster, sp. ind., Nicholson and Etheridge, Mon. Silurian Foss. Girvan Dist., Ayrshire, fasc. 3, 



pp. 325, 326, pi. xxi, figs. 9, 10. 



1914. Urasterella girvanensis, Schuchert, Fossilium Catalogus, Animalia, pt. 3, pp. 42, 44. 



1915. „ „ Schuchert, Bull. 88, U.S. Nat. Mus., pp. 167, 175, 180, pi. xxviii, fig. 5. 



M<tl, rial. — Only a mould of the oral surface of a single individual in Mrs. Gray's 

 collection (D. 9) was known to Xicholson and Etheridge, who remark : " In some 



1 (xm^is- = a spoke of a wheel.) 



