212 



SILURIAN TRILOBITES. 



probably it is soldered to the head, and in that case the eye (indicated by a notch at 

 a, fig. 8) may be absent, and the species a blind one. Collectors should search for the 

 species in its original locality, between Corndon Hill and Hope Mill, Shelve, Shropshire. 



A broad-oval species, two and a half inches long by two inches broad, of which length 

 the semicircular head is rather longer than the thorax, and shorter than the semicircular tail. 

 The axis is well marked out and narrow in the thorax ; very slightly so in the head and 

 tail. The semicircular head, with bluntish angles, is very slightly convex, but regularly so ; 

 and has short axal-furrows reaching a third up and nearly straight, marking out a glabella 

 so narrow as not to be nearly a third of the whole width. The facial suture is all but 

 marginal, and must cut the exterior edge about half-way up ; while the place of the eye, 

 indicated by a notch (the eye has not been found), is fully two thirds up the head. The 

 free cheek, therefore, as before said, must be very small, narrow, and triangular. 



Thorax well lobed, the axal furrows sharp, the axis subfusiform, narrower, but not 

 greatly so, than the nearly direct pleurae, which are flat as far as the fulcrum : this 

 is placed about one third out, and nearly equally distant in all the rings. The outer 

 portions of the pleurae are neither recurved nor deflexed, but run straight out ; and 

 they are oblique at the tips. 



Tail semicircular, the front edge straight or a little arched, with a very short narrow 

 conic axis, which indents about one fourth of the length of the tail. The upper angles of 

 the tail are very slightly truncated, and obliquely so. The surface is regularly and only 

 slightly convex. The fascia — seen best in Mr. Lightbody's specimen (fig. 8) and in 

 Sir Roderick Murchison's specimen (fig. 5) — is moderately broad, and of tolerably 

 equal width all round. 



Localities. — Llandeilo Flags proper (lower portion) ; near Hope Mill, north of the 

 Corndon Mountain, Shropshire (Mus. Geol. Soc, figs. 5, 6). Near Llanrian, Abereiddy 

 Bay, Pembrokeshire, (Mus. P. Geology, fig. 7, collected by Mr. Hughes ; fig. 8, in Mr. 

 Hicks' Cabinet). 



Sub-genus (?) 7 — Ill^engpsis, Salter, 1865. 



I can hardly believe this to be less than a generic group ; but prefer to keep it with 

 Illcenus for the present, as the Canadian sub-genus Hydrolanus so much tends to 

 connect it with the more ordinary forms. But while the anterior eye resembles Ectillanus 

 just described, the complete axal furrows show a tendency towards Bronteus, and the 

 grooved and pointed pleurae link it more with the ordinary forms of the Asaphida than 

 is exhibited in any other of the Illanus group. Perhaps Psilocephalus is the nearest 

 genus we can compare it with. And it is instructive to observe that Fsilocophalus preceded 

 IllcBnopsis in point of time ; and Illcsnopsis and Hydrolcenus, with complete furrows and 

 forward eyes, preceded Ectillanus ; and so far as I am aware, this latter sub-genus was 

 rather an earlier one than lllcenus proper. The group closed with the highest and 



