22 SILURIAN TRILOBITES. 



The tail is semicircular, gently convex, but with the axis depressed and flattened ; 

 it tapers slowly, has a blunt apex, and is marked by five or six segments. The side-lobes 

 have four or five abbreviated narrow furrows ; the upper ones are interlined, but all are 

 inconspicuous. 



Our larger figure (fig. 6) is from a doubtful locality in the Wenlock Rocks, but is 

 certainly British. It shows the internal cast of the head sufficiently well. The species 

 grows occasionally to a rather large size, as may be seen by this specimen, and others 

 in the cabinet of Mr. Hollier, of Dudley. But usually the specimens are not larger than 

 our other figures. P. sublcevis of M'Coy is a very obscure drawing, but the original 

 specimens do not differ from P. Siokesii. 



Localities and Geol. Ra?ige. — Llandovery Rock, Galway ; Ayrshire ; Haverfordwest, 

 Pembrokeshire. May Hill Sandstone, of Tortworth ; May Hill ; Malvern ; Shropshire. 

 Wenlock Rocks ; Abberley ; Malvern ; Dudley ; Wallsall ; North and South Wales ; 

 Dingle, West of Ireland ; Peebleshire. Ludlow Rocks ; near Leintwardine, Shrop- 

 shire ; Pentland Hills, Edinburgh. 



P. (Phacops) nudus, n. sp. PI. VI, figs. 19, 20. 



P. parvus, convexus, omnino P. Stolcesii simittimus, nisi penis profunde marginatis, 

 oculorum lentibus paucis, caudd rotundiore, lateribus injlatis, costis distinctioribus. 



It is difficult to define the fragments of this speeies, otherwise than by comparing the 

 parts with the better known and more perfect P. Stokesii, from which the species, a really 

 distinct one, differs in the following characters — the general form and the shape of the 

 glabella being extremely like in both cases. 



The cheeks are larger, more rounded, and less convex, rather suddenly raised, and 

 with a tumid space between them and the glabella (d). The lenses are far less numerous, 

 and have granules in the interspaces (/). The tail (fig. 20) is more oblong than a true 

 semicircle, transverse, blunt, and is more depressed. The axis is short, with an obtuse 

 flattened tip, and has six rings; the lateral lobes tumid, with four arched furrows 

 (including the uppermost one), and a very obscure fifth furrow. They do not nearly 

 reach the margin, and are faintly interlined. The tumid sides, being rather strongly 

 divided from the flattened axis, give a peculiar character to the tail, very different from 

 that of P. Stokesii. 



This new form has unexpectedly turned up in the collections made by the Irish 

 Survey in the wild district of the Dingle Peninsula. In the mountain of Cahirconree, 

 among slates of decidedly Upper Silurian date, occur some limestones, of whose age we 

 are not so clear, but containing the present species, which belongs to an Upper, and not a 

 Lower Silurian group. I have not much hesitation in referring them to the Wenlock, or 



