72 SILURIAN TRILOBITES. 



The neck-furrow strong. The forehead-lobe is small, generally equal in breadth, to the 

 rest, but no wider ; the other lobes equal in length and width. The axal furrows are deep 

 and strong ; in them, opposite the front furrows, are deep pits on each side in the cast, 

 which indicate the place for the processes, which join the corresponding projections of the 

 labrum. The cheeks are convex, wide-triangular, with a broad margin, and a narrow, 

 deep neck-furrow, continuous, as in all the genus, with the outer marginal furrows. The 

 free cheek is small ; the eye small, and placed midway on the most convex part of the 

 cheek ; the facial suture curving strongly inwards above the eye, and outwards and 

 upwards behind it. Spines short? and rather thick, directed backward. The whole 

 cheek is closely pitted, and the glabella and neck-segment sparsely covered with coarse 

 tubercles. 



The tail (Jrges planispinosus, Portl.) is very flat, and moderately transverse, with a 

 broad triangular axis of three rings, and a minute terminal appendix, which does not quite 

 reach the notched apex. The upper side-lobes or pleurae are largest and longest, broad- 

 lanceolate, with a short pleural groove, and directed outwards ; the second pair ovato- 

 lanceolate, and diverging but little ; the terminal pair of the same shape, but parallel, and 

 with a deep notch between them. All the lobes are adnate, but strongly separated by 

 furrows, and extend backward to about the same distance. Surface unknown. 



Localities. — Caradoc. Tyrone (figs. 7, 8) ; Ayrshire (fig. G), in the Craig Head Lime- 

 stone (Mr. J. C. Moore), occurring with Heliolites, Petraia, and Pleurorhynchus. (All 

 three figured specimens are in the Mus. Pract. Geology.) 



Ch. (Cheikurus) cancrurus, Sailer. PI. V, fig. 15 (and 16?). 



Ciieirurus GELA8INOSUS, M'Coy. Synops. Sil. Foss. Ireland, p. 44 (not of Portlvch). 



1846. 



— cancrurtjs, Salter. Decade Geol. Surv., No. 7, Art. 2, p. 11, 1853. 



— — Id. Siluria, 2ud ed., p. 538, 1859. 



Ch. ( Cheirurus) satis magnus, caudd tineas 20 lata, transversa, apice abrupte trancato 

 /jramorso ; axe lato, annulis quatuor subcequalibus, lerlio a quarto punctis binis remotis 

 solum separato ; lateribus spinis quatuor longis sub-par allelis, ad basin adnatis, transversis, 

 apicibus lente decurvatis ; basalibus utriusque lateris longo intervallo remotis. 



A most remarkable species, in which the four lateral lobes of the tail start horizontally 

 from the broad axis, instead of gradually converging beneath it, and leave its apex bare; 

 the breadth of this space being increased by the outward direction of the spines themselves, 

 which begin to curve downwards only when they have attained half their length. The 

 appearance of the perfect tail is just like that of a crab; prccmorsus might have been an 

 appropriate specific name. C. obtusatus, a Bohemian species, somewhat resembles this ; 

 but the spines are radiating, not parallel. None of Angeliii's Lower Silurian Swedish 



