ILLiENUS. 



189 



patience of classical readers for their unwieldy length. //. Bowmanni has the greater 

 range, for our fossil is confined to the Llandovery and ]\Iay Hill rocks ; but in the former 

 the two species frequently occur together, and their differences then become manifest 

 enough. The larger form has much less remote and less backward eyes, a stronger 

 trilobation, and a more tapering shape in the axis of the thorax ; the pleurae are more 

 abruptly bent backward ; and the tail longer in proportion to the width. 



It needs close examination to detect these proportional differences in the Illceni ; 

 but the habit and aspect would be alone sufficient, for 77. Thomsoni is very much less 

 convex, the thorax much longer, &c. These differences will appear more fully in the 

 course of description. 



A large oblong species, not less than six inches by three inches and a quarter ! 

 Rounded, but not obtuse, at either end ; regularly convex, not at all gibbous ; and with 

 the thorax strongly trilobed, the semioval head faintly so, and the tail scarcely at all 

 trilobate. The head appears to be scarcely at all longer than the tail, but both are longer 

 by one third than the thorax. The fine specimen in Pi. XXX, figured from Prof. 

 Wyville Thomson's Cabinet, shows these proportions. 



Head only gently convex, and divided pretty equally into glabella and cheeks by long 

 straight axal furrows, faint on the outside, but strong in the cast, with a pair of ovate- 

 lanceolate glands.^ The cheeks outside the facial suture are narrow and abruptly 

 depressed, but are level from the glabella to the eyes, the latter being large and promi- 

 nent, and placed at about two thirds the glabella's width away from it ; they are fully 

 their own length from the hinder margin, and being larger than in //. Bowmanni, of 

 course appear further forwards than in that species. The sculpture of the head, so 

 far as seen, consists only of short broken arched lines and a few puncta, except over the 

 front, where larger and more remote lines occur at rare intervals. We do not know the 

 actual front margin. 



Thorax of nine broad rings, greatly arched forwards on the axis, which even poste- 

 riorly is wider than the pleurae, and greatly so in front ; it is divided from these by strong 

 axal furrows. The pleurae rise a little as they leave the axis, and are gently convex 

 rather than flat^ towards the fulcrum, placed at less than half out in the hinder rings, and 

 at one third in front. Thence they bend strongly downward and backward, and are a 

 good deal thickened, as shown in the cast (PI. XXVIII, fig. '2). The tips are blunt- 

 pointed, not at all truncate as in fig. 7 (//. Boiomanni). 



Tail long, semioval, except for the strong re-entering curves and angles of the 



1 I can only suppose these oval spaces, frequently punctate-granulate, to represent one pair of the 

 curious gland-like markings which occur on so many segments of the head, body, and caudal axis in 

 various genera (p. 50, &c.). Prof. "W. Thomson does not believe them glands ; what are they? 



2 Illanus has apparently ungrooved pleurae in all the species. These specimens show the meaning of 

 this ; for the true position of the groove, which, distinct or not, exists in all Trilobites, is here quite at the 

 hinder edge of the pleura, and is visible internally in the two front rings. 



