8 



BEITISH FOSSILS. 



a sub-genus Phacops, while lie formed tlie section Dalmannia for tlie 

 more expanded forms, such as P. caudatus, P. Hausmanni, &c.* 



Professor Burmeister had already, 1843, regarded our species as a 

 synonym of P. macrophthalma, Brongn., and has repeated this 

 reference in his second edition, 1846. And Lieut.-Col. Portlock, in 

 his admirable work on the Geology of Tyrone, endeavoured to escape 

 from the difficulty by proposing a fresh name, P. Brongniarti, to 

 include Brongniart's and Murchison's species, as well as a new and 

 perfectly distinct form, discovered by himself ; thus adding inno- 

 cently to the confusion. In the Mem. Geol. Survey, 1848, 1 returned 

 to Milne Edwards' correct classification of these species, and de- 

 scribed both the English forms. Professor M'Goy has since confirmed 

 their distinctness, and we may now consider P. JDowningice as having 

 established its claim to rank as a distinct British species, highly 

 characteristic of the Upper Silurian rocks, and unknown, so far as 

 we are able to learn, in other countries. 



British Localities and Geological Position. — Caradoc Sandstone 

 to Ludlow Rock. — Garadoc Sandstone ; Moel Seisiog, and other 

 places near Conway and Llanrwst, North Wales (dwarf specimens). 

 Wenlock Shale ; Bryn Graig, &c., Usk ; and Slate Mill, Hasguard, 

 in South Wales. Wenlock Limestone ; west of Hereford Beacon ; 

 Ledbury ; Malvern Hills ; Dudley and Walsall, abundant. Lower 

 Ludlow and Upper Ludlow Rocks of the Abberley Hills. Upper 

 Ludlow ; Underbarrow and Benson Knot, Kendal ; Pont-ar-y- 

 Llechau, near Llangadoc, South Wales; Ludlow Rocks, Golden 

 Grove, and other places south of Llandeilo. 



Explanation of Plate I. 



Fig. 1, Phacops Downingice, of ordinary size ; Dudley limestone. (Collection of John 

 Gray, Esq.) At b, the outer termination of the facial suture is seen. 



Fig. 2. Do. ; a rolled-up specimen, same locality. (Collection of T. W. Fletcher, 

 Esq.) 



Fig. 3. Do. ; variety with very large eyes, each with about 180 lenses. Same locality 

 and collection. 



* As we think, however, that there are three distinct groups, we have adopted the term 

 Acaste for the present sub -genus, and left the species with inflated and lobeless glabella in 

 the section Phacops. In this latter view we have the sanction of the greatest authority 

 on trilobites, M. de Barrande, whose great work, just received from the publisher, 

 will long be the standard for reference. Otherwise we should have been unwilling to 

 disturb the nomenclature adopted by Professor M'Coy, who has given to the latter group 

 the new name Portlockia, reserving Phacops for those species which have the glabella lobes 

 distinct, but have not the expanded form or numerous tail segments of Dalmannia. 



