URASTERELLA THRATVENSIS. 137 



(3) A breaking up of the continuity From comparison with changes in the 



of the radial series Asteroidea of Section A. 



(4) A loss of symmetry in the central Ditto. 



plates of the disc 



The increase in size may be observed in the lineage commencing in the 

 Lower Ludlow U. ruthveni, var. leintwardinensis ; and progressing through the 

 stage U. ruthveni ; until we reach U. asperula of the Devonian. 



This same series shows a progressive increase in the comparative length of the 

 arm. The same increase can be observed in individuals of U. ruthveni as they 

 become more mature. U. asperula shows a loss of symmetry in the central plates 

 of the disc. A breaking up of the continuity of the radial series is seen in 

 U. ulrichi. 



It is possible that the long-armed Upper Ordovician British species U. thrai- 

 vensis may be descended from an American stellate form such as JJ. pulchella. It 

 obviously represents a distinct lineage from that arrayed round the later more 

 stellate Silurian U. ruthveni, although the distinctive characters are very minute. 



1. Urasterella thraivensis, n. sp. Plate VIII, figs. 1 — 3; Plate IX, figs. 1, 2; 

 Plate X, fig. 1 ; Text-figs. 81 (p. 129), 83—89. 



Material. — Six specimens are known, all moulds in sandstone, in Mrs. Gray's 

 Collection from Thraive Glen. D. Ill shows the structure of the apical surface of 

 the disc and portions of three arms; D.35 and its counterpart give rather distorted 

 views of the disc and proximal portions of three arms ; D. 254 shows excellent 

 views of the mouth-parts and of the oral side of the arm; D. 230 and D. 237, and 

 their counterparts, enable one to obtain very various views of isolated plates as 

 their component ossicles have been considerably displaced and scattered. D. 40 

 with its counterpart is probably a young form of the species. 



Specific Characters. — Ridges on adambulacralia set well in the middle of the 

 oral surface. 



Apical Surface (Plate IX, fig. 1 ; Text-fig. 88). — The structure of the central 

 portion of the disc and of the arms can very well be made out in casts from D. 111. 

 A plan of the ossicles is given (Text-fig. 88). The general arrangement of the 

 plates has already been referred to (p. 129). The quinque-radiate symmetry of the 

 disc is very striking. There is a centrale surrounded by five primary radialia. 

 Each of these is a prominent plate with a distinct longitudinal ridge. Only one 

 interradial is present, namely, that in the madreporic interradius. The madre- 

 porite is quite small, distinctly smaller than a primary radial. The remaining 

 radialia are stout somewhat squarish plates which run without interspaces down 

 the exact centre of the arm. Like the primary radial they have a distinct longi- 



