214 



SILURIAN TRILOBITES. 



rather strong upper furrow. The fascia complete, narrow, and equal all round. I know 

 no fossil that can be a very close ally of this, unless it be the Illcenus conifrons of 

 Billings, figured by that able palgeontologist from the Trenton group of Canada. Perhaps, 

 as suggested by me in the * Survey Memoirs,' vol. iii, //. conifrons may be of this 

 genus ; but it differs in so many respects that I have even ventured to constitute it a 

 (Ustinct sub-genus. II. clavifrons^ Billings, and also //. arcturus, Hall, from the Lower 

 Silurian rocks of New York and Canada, have the eye forward, but not nearly so much 

 so as ours. I beg to dedicate the species to my accomplished friend and fellow-worker 

 Prof. Wyville Thomson, whose help to me, both as a thorough naturalist and the owner 

 of a choice cabinet, is invaluable. 



Locality. — Arenig Rocks; Ritton Castle Mine Works, near Bogmine, Shelve, Shrop- 

 shire (Mus. P. Geology) ; in a lead-coloured soft shale (not slate), which abounds in well 

 preserved and rare fossils. 



More profitable ground to work than this locality I hardly know in Britain. Two 

 species of jEglina, the rare Trinucleus Murchisoni, Agnostus Morei, an undescribed 

 Asaphus or Ogygia, and a new Cheirurus, are found there. With them occur the Twin- 

 graptolites of two species ; the characteristic Oholella plumbea ; bivalve Shells ; a Theca ; 

 two species of Orthoceras ; Bellerophon, &c. This and the neighbouring mine-work of 

 Cefn Gwynlle are the best localities for well preserved fossils in the Arenig or Skiddaw 

 group ; and it was by finding fossils there that I was enabled to establish the existence in 

 1857 of a distinct fauna for this formation, which had been previously named by Prof. 

 Sedgwick. It is a more extensive group than the Llandeilo proper, and has its repre- 

 sentative abroad in the fossil-bearing slates of Brittany (Angers, &c.), so celebrated for 

 their great Trilobites. The fossiliferous Lower Silurian schists of Oporto are probably 

 of the same age. Collectors of Trilobites may therefore expect to find in the lead- 

 bearing rocks of the Shropshire district some of the curious Trilobites described by De 

 Verneuil, Sharpe, Rouault, and Barrande, — such as P/aco^ana Zippei and PI. Tournemini. 



I think I shall be doing good service to our science by pointing out where a rich 

 harvest of new forms may be expected, and I hope our friends will send me (for exami- 

 nation at least) some of the first-fruits. 



The shales and sandstones west of the Stiper Stones ; the slates under the Arenig 

 and Cader Idris porphyries ; those of Ty-obry in the pleasant Tremadoc district ; the 

 lower and larger half of the Skiddaw Slate ; and the prolific vertical shales of White- 

 sand Bay and Ramsey Island, St. David's, should all be thoroughly worked by those who 

 wish to add new forms to our Trilobitic faunae. 



I take this opportunity also to include in the Arenig formation the puzzling Budleigh 

 Salterton pebble-bed. In former pages of this work it was provisionally referred to the 

 Llandeilo rocks. This pebble-bed (the record of vanished rocks hard by in the old 

 time) and the qunrtz-rocks of Gorran Haven, S. Cornwall (which are relics of the 



