﻿HOMALONOTUS. 



107 



internal ridge.) 1 The pleurae are themselves regularly convex and not abruptly bent 

 down. Their facets are large, their ends expanded, and not much bent forward. The 

 pleural groove is narrow. 



The tail is semi-oval, its end obtusely rounded, its margin all round slightly concave ; 

 the anterior edge not greatly curved, and the facet very strong. The axis is gradually 

 tapering, and reaches four fifths of the length ; and it is then continued by a conical 

 appendage nearly to the margin. It is not very prominent or strongly marked out, but 

 is nearly as wide as the side-lobes, with eleven or twelve segments, of which the two 

 anterior are much stronger than the rest ; sides gently sloping, with about seven broad 

 flat ribs, separated by narrow sulci, the two anterior of which are disproportion ally deep, 

 the rest very shallow. These side-furrows are straight, and have a marked interval 

 between their origin and the furrows on the axis. The contrast between the two deep 

 upper furrows of the tail and the remaining faint ones is very striking, and suggested 

 the name. 



Variations. — The young have the head-furrows somewhat stronger. The tail-axis 

 varies a little in width, and has nine to ten or eleven rings ; and the sides have stronger 

 or fainter furrows. 



Trimerus platypleurus of Green, ('Silliman's Amer. Journ./ vol. xxxii, p. 169), nearly 

 resembles ours in the caudal shield, but differs in its proportionate length and breadth ; 

 in the short-conical form of its axis, and the stronger lateral furrows. H. Jacksoni, Green, 

 is another allied American form. 



Localities. — Arenig group, Tremadoc, N. Wales. Llandeilo flags, Anglesea? 

 Caradoc sandstone, everywhere in N. Wales and Shropshire. 



Homalonotus Sedgwicki, n. sp. Woodcut, fig. 25. 



Fig. 25. 



Homalonotus Sedgwic&i, n. sp. From Caradoc Rocks in Westmoreland. 



1 The condition of the fulcrum is rather complicated in Homalonotus, from its coinciding with the 

 axal line, and it is difficult at first to interpret the casts of exterior and interior. 



