BRITISH FOSSILS. 



3 



British Localities and Geological Range — Llandeilo Flags only. — 

 Abundant in the Bala limestone at Rhiwlas, at tlie foot of the lake, and 

 also at other localities in the immediate neighbourhood both north and 

 east of the lake (Coll. of Geol. Survey). It appears, however, to 

 be but local, and its place is taken through many other parts of North 

 Wales, and all through South Wales, by the Bowmanni. It pro- 

 bably occurs in the lower Silurian limestone of Wrae, near Broughton, 

 Peebleshire ; some fragments were found there by Mr. James Nicol 

 (Geol. Journ., vol. iv., p. 206), which are with doubt referred to this 

 species, as also is another portion of thorax found in the limestone of the 

 Stincher river, Ayrshire, by Mr. Carrick Moore (ih., vol. v., p. 13). 



Explanation of Plate II. 



Fig. 1. Nearly perfect specimen. Ehiwlas, Bala ; in which locality all, except figs. G, 

 7, were found. Fig. 1*, side view of ditto. 



Figs. 2, 2*. Front and side views of a rolled-up specimen ; the want of gibbosity in the 

 head is shown in the side view. 



Fig. 3. Dissected figure, a, central lobe ; h h, wings, upper side ; they extend further 

 inward on the under side ; c, axis of a middle thorax segment ; d d, fulcra ; e e, fulcra 

 of tail. 



Fig. 4. A separate tail, showing the incurved under portion, d, where the upper crust is 

 broken off. 



Fig. 5. Rostral shield, often found separate. 



Figs. 6, 7. Tails of young specimens, showing the further extension of the furrows at 

 that age. Llechwedd ddu, Bala. 



Fig. 8. Variety with narrow axis. The tail has been compressed from below, and 

 appears too small. Rhiwlas. 



British species of Illcenus proper. 

 * With 9 body-rings. 



1. /. Bowmanni, Salter, Mem. Geol. Survey (June, 1848), vol. ii., part 1, pi. 8, f. 1-3; 

 /. centrotus, Portlock (1843), Geol. Rep., pi. 10, f. 3-6, and probably f 9. 



/. copite magna, angulis posteriorihus obtusis ; oculis remotis, ad marginem occipitalem 

 approximaiis ; thorace caudd hreviore. 



Locality. — Llandeilo flags. North and South Wales ; Kildare, Ireland. 



** With 10 body rings. 



2. /. Davisii, as defined in our preceding description. 



3. /. Portlockii, Salter; /. crassicauda, Portlock (1843), Geol. Rep. Tyrone, &;c., pi. 10, 

 f. 7, 8 [Thaleops ovatiis, Conrad ; Hall, Paleont. New York (1848), pi. 67, f. 6 6. ? 



/. capite magno, cauda parva, suhirigona ; oculis remotis [prominidis ?] ; thorace seg- 

 mentis 10, axi lato, pleuris ahrnpte deflexis et reflexis ad fulcrum, quod ad dimidium pleurcs 

 uniusquisque positum est ; caudd depressa, suhtriangidari, angulis extremis late truncatis. 



That this neat species is not /. crassicauda is very clear ; the shape of the tail, and the 

 abrupt backward bend of the pleurae, almost at right angles, distinguish it easily ; the 

 fulcrum is as far outward in the first segment as the last, an unusual character. We have 

 not seen specimens of /. ovatus (Conrad); Hall's figure of that species (loc. cit., pi. 67, 

 f. 6 h) much resembles ours in the tail ; but if his two specimens be correctly drawn, he 



