186 



SILURIAN TRILOBITES. 



front to back the curvature is similar, but more abrupt in front, where the forehead 

 overhangs. 



Thorax about half the length of the head, and about three-fourths the length of the 

 tail ; of 9 rings. The axis gently convex, broader than the lateral lobes, and tapering 

 but little backvv^ards ; marked out by strong but not deep axal furrows from the pleurae. 

 These are flat as far as the fulcrum, v^^hich is at one third anteriorly, and at one half 

 posteriorly. From this point the pleurae are bent gently down and a little backwards ; 

 the front ones more so than the hinder ones, which last are nearly direct. 



Tail regularly and gently convex, half a broad oval ; the length being two thirds the 

 breadth. The upper angles are strongly truncated ; the axis, which is more than one 

 third the breadth, being only marked out by broad indentations in the upper part, and 

 having no true axal furrows. The tail-margin is neither obtuse nor recurved, but regularly 

 and evenly declines to the smooth edge. Caudal fascia concave,^ rather narrow, especially 

 towards the upper angles, broader below, and not indented by the axis internally. 



Variations. — In young specimens the glabella is narrower in proportion (PL XXVIII, 

 fig. 8 ; and see also Portlock, PI. 10). The thorax too is longer in proportion and more 

 quickly tapering behind ; the tail is flatter, and so short as to be less than a semicircle 

 (see also fig. 9). Such variations in the young state are common among the species of 

 this genus. Rarely, as in PI. XXVIII, fig. 11, the glabella-furrow^s are longer and more 

 connivent, and more recurved above ; fig. 6, again, same plate, represents a May Hill 

 Sandstone specimen, in which these furrows are shorter than usual. Some specimens 

 have them straighter than others, and they vary a little in depth. 



Illanus latus, of M'Coy, is, I think, only a compressed specimen. We have several 

 such forms from Pembrokeshire and elsewhere. It is figured in p. 215, woodcut 54. 



Our fossil has been referred both by Gen. Portlock and Prof. M'Coy to the Swedish 

 form, //. centrotus. I cannot clearly make out why ; for the distinction of rounded 

 (instead of spinose) angles was clearly pointed out by myself, in the Appendix to the 

 Woodwardian Synopsis ; and I communicated freely with Prof. M'Coy. 1 have re-examined 

 the foreign specimens, and am convinced the species are only allied, not identical. The 

 position of the eye at once separates our fossil from //. Davisii ; these two hardly ever 

 occur together, //. Davisii requiring, to all appearance, a purer element than the more 

 common //. Bowmanni, which could live on mud, sand, gravel, or any kind of sea-bottom. 

 It, however, seems to have preferred a calcareous sea-bed when it could get it. 



Localities. — Llandeilo Plag, or Lower Bala, rare. Knockdolian, Ayrshire {TFoodiv. 

 Mus.) ? Everywhere in Caradoc or Bala Rocks, N. and S. Wales, Westmoreland; S. 

 Scotland ; North, East, West, and South Ireland. A few localities may be given : Bala, 

 abundant, especially east of the lake (/. Davisii, west and north) ; Dinas Mowddwy ; 

 Llanfyllin ; Llanfairynghornwy, Anglesea ; N. W. of Llandeilo ; Shoal's Hook, &c., in 



' That i.s, rt (lertecl closely for its antprior half upon the upper crust ; most of the species have it convex, 

 I. e., concave upon the cast. Probably the difference is due to greater flexibility in the membranous fold. 



