82 BRITISH CAMBRIAN TRILOBITES. 



of the glabella, curving outwards and backwards to the eye and behind the eye 

 bent abruptly outwards and finally backwards to the posterior margin. Fixed 

 cheeks at the eye about two-thirds the width of the glabella, behind the eye 

 expanded to about one and a half times that width. Free cheeks with the outer 

 margin somewhat angulate towards the front ; from the angle springs a long strong 

 spine which curves backwards. Hypostome tongue-shaped, parallel-sided, with an 

 oval convexity in the proximal portion. 



Thorax consisting of ten segments, rapidly decreasing in width from the fifth 

 segment. Axis rather less than the pleura3 in width, with straight sides, narrowing 

 posteriorly, each segment bearing a median tubercle or spine; the axis of the tenth 

 segment somewhat triangular, produced into a long straight spine directed back- 

 wards. Pleura3 straight and horizontal nearly to their extremities, where they are 

 bent slightly downwards, rapidly decreasing in length after the fifth segment, ex- 

 tremities produced into long spines, which in the first three or four segments are 

 directed outwards and only slightly backwards, but in the later segments are 

 progressively more and more strongly bent backwards ; pleural grooves broad and 

 dee}), especially near the outward end, terminating at the commencement of the 

 spines. 



Tail very imperfectly known, apparently rather small, triangular. Axis wide, 

 forming almost half the total width, lateral lobes narrow. 



Dimensions. — The cranidium varies from about 2'5 mm. to 5 mm. in length and 

 from 6 mm. to 12 mm. in width at the posterior margin. The thorax shown in Plate 

 IX, fig. 1, must have been rather less than 10 mm. long when complete, and that 

 shown in Plate IX, fig. 2, rather less than 7 mm. These appear to be fairly average 

 examples of the long and the broad forms when fully grown. 



The tubercles on the axis of the thorax are not always clear, especially on 

 internal casts. They appear to have been the bases of short spines, for in ex- 

 ternal moulds the corresponding depressions penetrate some depth into the matrix, 

 and the solitary thoracic segment figured in Plate IX, fig. 3, shows a definite spine 

 rising from the axis. 



In Plate IX, fig. 1 it Avill be seen that two spines appear to spring from the 

 axis of the tenth segment, one straight and the other curved. Such an arrange- 

 ment looks abnormal, and it is natural to suspect that one of these spines may be 

 a fragment of some other individual accidentally brought into its present position. 

 A close examination with the microscope proves, however, that both spines are 

 definitely connected with the axis. The specimen shows the interior of the test, 

 and in the normal position the curved spine would be beneath the straight one. 

 The straight spine was probably a sheath in which the other usually lay. 



There appear to be both a long form (Plate IX, fig. 1) and a broad form 

 (Plate IX, fig. 2) of this species, and in the former the thoracic spines are much 



