ILLvENUS SHALLOCHENSIS. 69 



Thorax of nine rings, about two thirds the length of head-shield. Axis wide, 

 gently convex, nearly cylindrical, scarcely decreasing at all in width posteriorly. 

 Pleuras with an inner straight horizontal portion, and an outer (extra fulcral) 

 portion bent downwards nearly at right angles, and also bent backwards at about 

 45° to the inner portion. Inner portion increases in length from head-shield to 

 about middle of thorax, where it is nearly half the total length of pleura, but 

 behind this point it again decreases slightly to pygidium. (In the Shalloch Mill 

 specimens the shell surface of the axis and pleurae shows a few fine irregular 

 transverse wrinklings or raised lines.) Pygidium broadly oval, wider than long, 

 very gently convex, but rather steeply and suddenly bent down near the posterior 

 margin. Axis wide, about one third the width of whole pygidium, slightly 

 projecting on front margin, without independent convexity or axial furrows, and 

 only indicated by pair of short shallow marginal depressions. Fulcrum weak, 

 distant from the axis less than half the width of latter. Lateral angles truncated 

 at about 50° to front edge; faint shallow groove behind fulcrum. Narrow median 

 groove behind axis running back across the fascia to the margin. Doublure or 

 fascia consists of narrow marginal convex portion (concave in casts) and broad 

 concave portion (convex in casts), and both portions are marked by a few rather 

 irregular concentric striae. 



Surface of shell of pygidium punctate as head-shield. (The better preserved 

 Shalloch Mill specimens also show some transverse irregular broken lines.) 



Dimensions. — 



Specimen from Thraive Glen. From Shalloch Mill. 



I. II. 



Length of head 



. 25-0 



Width of „ 



. 28-0 



„ of glabella . 



. 13-0 



Length of ,, 



. 11-0 



Width of fixed cheek 



6-0 



,, of free cheek 



. 9-0 



„ of thoracic axis 



. 14-0 



Length of thorax . 



. 14-5 



,, of pygidium 



+ 22-0 



Width of ' „ . 



. 28-0 



23-0 21-5 



28-0 2(3-5 



Remarks. — This new British species of Illsenus is represented by a considerable 

 number of fragments (head-shields, cpistomes, free cheeks, and thoracic rings) 

 and by a few detached but nearly perfect pygidia from Shalloch Mill in Mrs. 

 Gray's collection. The thick black shell is preserved for the most part in all these 

 specimens. A remarkably fine and complete individual from Thraive Grlen showing 

 the same characters but in a different state of preservation has been lent me by 

 Mr. A. Macconochie. From the latter the above complete description has been 



