24 BRITISH CAMBRIAN TRILOBTTES. 



The tail of this species appears to be identical with that figured by Linnarsson 

 as the tail of A. sidenbladhi, but the head is decidedly different. Mr. W. G. 

 Fearnsides, who found the specimens figured on Plate II as A. calvus and A. 

 sidenbladhi, informs me that the former occurs at the base of the Asaphellus flags 

 and the latter at the top. In Scandinavia the thickness of the deposits is much 

 reduced and the two should occur within a few feet of each other. 



Horizon and Locality. — Tremadoc : base of Asaphellus flags near Nant rhos ddu, 

 Aronio-. 



21. Agnostus dux, Callaway. Plate II, fig. 19. 



1877. Agnostus dux, Callaway, Quart. Journ. Geol. Soc, vol. xxxiii, p. 665, pi. xxiv, fig. 3. 



For the following description and notes on the affinities of this species I am 

 indebted to Mr. Frank Raw of the University of Birmingham, who has made a 

 special study of the Shineton Shale fauna. 



" Head roughly circular in outline, somewhat quadrate, widest in front of the 

 centre, narrowing slightly posteriorly, genal angles obtusely pointed. Marginal 

 rim almost flat round the front half of the shield, broadest at the anterior corners. 

 Cheeks smooth, convex, nearly uniform in width round the front of the axis. 

 Glabella about half the length of the head and one third its width at the back, 

 narrowing in front; rounded in front and behind, convex, bilobed, the anterior 

 lobe occupying about one third of the length ; basal lobes very small, lateral. 



" Thorax : central division of the axis wide, being erpial in the anterior segment 

 to the articulating knob together with the pleura on each side ; narrower in the 

 posterior segment; pleurae curved forwards, posterior lateral edges semicircular; 

 posterior longer than the anterior pleurae. 



" Tail a quadrilateral oval, of the same width as the head, but somewhat shorter, 

 lateral margins continued into short spines ; limb large, wider behind, with a fairly 

 flat margin and a shallow depression running round the limb at one third the width 

 from its edge. Axis wider than that of the head but somewhat shorter, rounded 

 behind, slightly broadening in front; anterior segment widening into a lobe on 

 each side, second segment extending Forwards bet ween these but not reaching the 

 anterior border, bounded behind by a faint transverse furrow, anil bearing a blunt 

 longitudinal tubercle rising in height to the back of the segment; posterior segment 

 a little mere than half the length of the axis. 



"Dimensions of Callaway's type specimen, the largest seen by the writer: 

 Total length, 8 mm. ; head, length 3"8 mm., breadth 4*1 mm.; tail, length 3"5 mm., 

 breadth I' 1 mm. 



"Affinities. Brogger 1 compares .1. i/n.r with .1. sidenbladhi, which occurs 

 1 Die Silur. Etagen 2 und :!, \>. 1 14. 



