44 SILURIAN TRILOBITES. 



the outer pair continuous as a marginal furrow, the inner very oblique. The apex is 

 angular, but blunt. 



Localities. Caradoc of Desertcreat, &c., Tyrone. (Mus. Pract, Geology). 



Phacops (Chasmops) Bailyi, n. sp. PI. VII, figs. 21 — 24. 



P. (Chasmops) satis magnus, capite subangulato ; glabella dilatatd, lobis omnibus distinctis; 

 cauddque (associatd) rofundatd midtiannulatd. Glabella longa elevata tuberculosa lobo 

 antico rhomboideo transverso, lobis lateralibus inaqualibus, supremo magno rotundato trigono, 

 secundo paullum abbreviato, basali majori, cervicali expanso. Oculi elevati. Cauda semi- 

 oval is, apice rotundato obtuso ; axe longo conico 10 — W-annulato ; costis lateralibus 11 

 plants, arcuatis, lineisque intermediis distinctis ad apicem una cum primariis connatis. 



It is scarcely doubtful that this is a Chasmops rather than Odontochile. It certainly has 

 many affinities with P. truncato-caudatus, to which Mr. Baily referred it in the Irish col- 

 lection. He has judiciously preserved every fragment, and in this way we have been 

 enabled to reconstruct the species.* 



The head is rather elongate, the glabella somewhat depressed and widely clavate, broad 

 above the wide forehead, and actually overhanging the eye, and narrowed behind ; all the 

 lobes distinct and somewhat radiated. The forehead-lobe is transverse and rhomboida], 

 separated by an arched furrow from the rounded, triangular, upper lateral lobe, and this 

 by an oblique downward furrow from the much smaller mid-lobe, which is triangular in 

 the reverse direction. The basal lobes are large, and tumid at the sides ; the cervical lobe 

 is wide. The eyes prominent, and placed rather forward, on a somewhat sharply elevated 

 cheek. The margin of the cheek narrow, and not strongly defined. The facial suture 

 runs in a broad ogive some distance within the front margin, which is slightly angulated. 



Of the tail we have one external and three interior casts, which show it to have been a 

 large semioval plate, broader than long, but rather abruptly narrowed behind, and with a 

 rounded obtuse end. The axis is narrow and conical (but broader than in P. truncato-cau- 

 datus), reaches five-sixths the length of the tail, and has ten or eleven rings at least. The 

 axal furrows are deep, and the sides are convex, and scored by eleven narrow sharp furrows ; 

 the ribs between which are flat and interlined all along, much in the same way as in P. 

 macroura. The upper ribs are duplicated very strongly. The margin is very narrow. 



It is very like P. macroura, above figured, and differs from P. truncato-caudatus, 

 fig. 13, both in the shape and proportions of the glabella-lobes, and in the much narrower 

 segments of the tail, and it does not need comparison with other British species. 



Locality. — Caradoc slates of Tramore, Co. Waterford. (Mus. Irish Industry, Nos. 

 B 647, 650, 652.) 



* Many species are lost from want of care in the collocation of fragments of specimens. Very often, 

 indeed, in slate-rocks, the species must be truly reconstructed from the disjecta membra, and with 

 judgment this may generally be safely done. 



