PHACOPS (PTBRYGOMETOPUS) RETARDATUS. 



51 



sparingly distributed subcquidistant tubercles of subequal size. Surface of cheeks 

 very faintly tuberculated. 



Thorax decreasing in Avidth posteriorly, composed of eleven segments. Axis 

 moderately convex, rather less than one-third the width of thorax at front end, 

 subcylindrical, tapering very slowly to pygidium. Axial rings arched slightly 

 forwards in middle and at sides, with weak lateral nodes. Pleuree somewhat 

 flattened, with strong diagonal furrow; fulcrum situated at less than half their 

 length, with inner half horizontal and outer half strongly arched downwards 

 and slightly backwards to end in short falcate point. 



Pygidium parabolic to semioval, nearly or (juite as long as wide, strongly 

 arched down at sides ; apex emarginate ; no distinct border. Axis moderately 

 convex, conical, about one-third the width of pygidium at front end, tapering 

 posteriorly to bluntly pointed end, annulated with 18 — 20 rings, of which the 

 first 12 — 14 are distinct and similar in character to those of thorax with lateral 

 swellings, and arched forwards in middle. Pleural lobes composed of 14 well- 

 marked flattened regular pleurae corresponding to the distinct axial rings, 

 decreasing successively in size posteriorly, arched gently and regularly backwards, 

 marked by fine weak median furrow, in which lies a row of small pits, and with 

 a faint row of small widely spaced obscure tubercles on each side of it. Front 

 margin of pygidium furnished with flattened half pleura on each side, and 

 articulating ring on axis. Interpleural furrows strong, regular, of uniform depth, 

 continued nearly to margin of pygidium. 



Dimensions. — 



II 



III 



Length of whole trilobite 



37-00 



mm. 



. 



— 



,, of head-shield 



11-75 



55 



8-00 mm. 





Width of 



20-25 



55 



. 17-00 „ 





Length of glabella . 



'. 9-00 



55 





10-0 mm. 



Width of „ at base . < 



•. 5-00 



55 





5-0 „ 



Height of eye from base 



4-75 



55 







Length of pygidium 



12-50 



55 



. c. 6-50 „ 





Width of 



14-00 



55 



. r. 9-50 „ 



— 



,, of axis of pygidium 



5-00 



55 



. 4-00 „ . 





,, of axis of thorax at 











front end 



6-00 



55 



5-50 „ 



— 



Length of thorax . 



12-25 



55 



. 10-00 „ 



— 



Remarlis. — The above described species is based on three nearly complete 

 individuals and one detached imperfect head-shield. In characters it is almost 

 indistinguishable from P. trancato-caudatus, Portlock,^ as defined by Salter," but, 



1 Portlock, ' Geol. Rep. Londonderry,' i^. 281, pi. ii, figs. 1—4. 

 3 Salter, ' Mon. Brit. Trilob.,' p. 42, pi. iv, figs. 13—15. 



