42 SILURIAN TRILOBITES. 



P. (Chasmops) amphora, Salter. PI. IV, fig. 16. 



Phacops (Dalmania) amphora, Salter. Decade Geol. Survey, vii. Art. 1, p. 12. 1853. 



P. {Chasmops) caudd magna biunciali elongatd, convexissimd, fere semicylindricd. 

 sulcis axalibus fere obsoletis. Axis latus nee eminens, marginem angustum inflexum hand 

 atlingens, in annulos 16 subplanos divisus, apice obtuso. Later a valde curvata deflexa, 

 costis 14 — 1 5 planis, sulcis acutis separantibus, costd qudque lined mediand lavi elevatd 

 sub cortice tenui impressd; apice obtuso {emarginato ?). 



This species must be closely allied to the preceding. It differs only in proportions, but 

 markedly so, being greatly more convex in the only portion we know — the tail ; it is 

 almost semicylindrical in shape, and the axis much less distinct from the sides than in P. 

 macroura. It is also, but somewhat more remotely, allied to P. truncato-caudatus. 



The large tail, which must have been fully two inches long, is but a little more than this 

 in breadth. It is more than half a cylinder, the curvature being remarkably regular and 

 even, the axis scarcely projecting beyond the general convexity, and scarcely divided from 

 the sides by any depth of axal furrow. The axis is broadly conical, and below rounded 

 off towards its apex ; it reaches fully five-sixths the whole length of the tail ; and is made 

 up of sixteen rather flat rings. The four or five front ones 'show a false articular surface, 

 distinctly ; the real articular surface is very large and distinct. 



The side-lobes are in reality half as wide again as the axis, but on a front view do not 

 appear so, owing to the great convexity ; the surface is curved steeply down, and is scored 

 by fifteen deep sharp furrows, separating sixteen prominent flat ribs, much wider than the 

 furrows. Each rib has an interrupted chain-like dividing line down its middle. The same 

 occurs, somewhat exaggerated, beneath the crust, on the cast. But the perforations here 

 are so arranged as to leave a crest-like "ridge. The dividing lines extend nearly to the end 

 of the ribs, which cease rather suddenly near the steep, almost vertical or inflexed, marginal 

 rim. 



The shell or crust of this species is remarkably thin. Except some species of Homalo- 

 notus, I do not know any kind of trilobite which has so cylindrical a form. 



Locality. Caradoc limestone of Grug, N. of Llandeilo, Carmarthenshire. Collected 

 and presented to the Mus. Pract. Geology, by Mr. Williams, of Llandeilo. 



P. (Chasmops) truncato-caudatus, Portlock. PI. IV, figs. 18 — 15. 



Phacops truncato-caudatus, Portlock. Geol. Rep. Tyrone, 1. c, pi. ii, figs. 1 — 4, 



1843; Paradoxides Bucephali, ibid, (labium only), 

 pi. i, fig. 8. 



— Salter. In Morris, Catal., 2d ed., 1854. 



— (Dalmania) Id. Decade II, Geol. Survey, Art. 1, p. 



7, 1849 ; (exclude the synonyms of D. affinis, for 

 which see P. macroura). 



