10 BRITISH CAMBRIAN TRILOBITES. 



posteriorly, with a median tubercle, basal lobes small. Cheeks smooth, or some- 

 times with faint radiate markings, widest about the position of the anterior lobe, 

 rather narrow in front of the glabella, where they are separated by a groove which 

 is sometimes obsolete. 



Thorax : axis divided into a central portion and two lateral knobs, all three 

 approximately equal in size. 



Tail rounded, with narrow margin, which in some specimens bears very 

 small spines posteriorly. Axis wide, forming nearly half the width, and reaching 

 nearly to the posterior margin. First segment represented by two lobes at the 

 anterior corners ; second segment hexagonal, the anterior angle separating the 

 two lobes of the first segment, the posterior angle raised into a median tubercle ; 

 posterior segment large, rounded posteriorly, with parallel sides. Lateral lobes 

 very narrow, especially posteriorly. 



Head- and tail-shields up to 3'5 mm. in length and width. 



This variety is easily distinguished from the typical A. pisiformis by the great 

 width of the axis of the tail, and by the fact that the anterior segment of the axis 

 is represented only by two lobes at the anterior corners of the axis. In this latter 

 character and in the general form of the pygidial axis, the species approaches 

 A. rudis and A. sidenUadhi, of which it appears to be the ancestral form. From 

 both of these it is distinguished by the greater length of the glabella, while from 

 the latter it is separated by the fact that the pygidial axis nearly reaches the 

 posterior margin. The axis is also much more swollen than in any of the species 

 mentioned. 



Although Belt's figure is rather rough, there can be no doubt that it is the 

 same form as that described by Tullberg under the name socialis. 



Horizon and Localities. — Lower Lingula Flags : Maentwrog Falls ; Mawddach 

 Valley ; Dolgelly ; Trefgarn Bridge and Leweston Old Mills, Pembrokeshire ; 

 Chilvers Coton (Stockingford Shales). In some cases the horizon is given as 

 Menevian, in others Upper Lingula Flags, but these determinations may be 

 doubted. 



7. Agnostus trisectus, Salter. Plate I, figs. 15, 16. 



1864. Agnostus trisectus, Salter, Mem. G-eol. Surv., Brit. Org. Remains, dec. xi, pt. i, p. 10, pi. i, 



fig. 11. 

 1864. Agnostus princeps, var. a ornatus (pars), Salter, ibid., p. 4, pi. i, figs. 4, 5. 

 1868. Agnostus trisectus, Belt, Geol. Mag., vol. v, p. 11. 



1878. Agnostus turneri, Salter MS., Cat. Camb. & Sil. Foss. Mus. Pract. Geol., p. 12. 

 1880. Agnostus trisectus, Tullberg, Agnostus-arterua, p. 24, pi. i, fig. 13 a, b. 



Head rounded, with narrow margin separated by a rather wide furrow. 



