2 



BKITISH FOSSILS. 



Beneath the head the hypostome is continuous '(fig. 2 a) and 

 tumid, without rostral shield or any suture, and the convex labrum 

 has rather a broad base, as in Illamus, 



The thorax is convex, and has a wide and much tapering axis, 

 broader than the sides. It is greatly wider in front than its pleurse, 

 and behind is not quite so wide as these. Its rings are gently 

 arched, and the third one bears a pair of those enigmatical tubercles 

 which are of so common occurrence along the^ central lobe of Trilo- 

 bites.* The front pleur83 are very short, and the hinder elongated. 

 They are more curved forward than our figure indicates, at least in 

 the central and hinder rings. The fulcrum is near the axis, the 

 groove broad, not deep ; the apices truncate, and a little pointed 

 behind. The facet is long and narrow. 



The tail is truly triangular, and except that the apex is rounded 

 off, would be an equilateral triangle, deeply and strongly margined 

 all round, and with a narrow conical axis which reaches two-thirds 

 and rather more of the length. Its tip is rounded, and it is marked 

 by a single broad ring at the base, and very faint traces of two or 

 three others. 



Variations. — Young specimens, which I have seen since the 

 plate was engraved, in the choice cabinet of Mr. H. W. Edgell, show 

 the metamorphosis. In a specimen which is barely two lines in 

 length, the number of rings to the body is only four, and the fourth 

 ring is scarcely separable from the caudal shield. The axis of the 

 latter is narrower than in full-grown specimens ; but I do not see 

 much other difference. 



This is one of the most conspicuous species of the genus, and in great 

 plenty in the black slate of one locality. It differs so markedly 

 from all the other species, in the triangular tail with a prolonged 

 axis, that it is unnecessary to compare it with any. It appears to 

 have grown to a less size than grandis next described. 



Locality and Geological Position. — Lower Llandeilo Flags 

 (Arenig group), Cefn Gwynlle, in the district west of the Stiper 

 Stones, Shropshire ; in black slate, abundant. My friend. Dr. A. 

 Fritsch, of Prague, tells me he has found the same species in the 

 Lowest Llandeilo beds (d. I) of Prague. I suppose it a closely allied, 

 not identical, form. 



* They have been specially noticed in my memoir on the Phacopidce, Palseont. Trans- 

 act., vol. for 1862, p. 52. And they are conspicuous under various forms in Encrinurus, 

 Cheirurus, Sao, and a host of other genera. Probably they indicate the places of 

 cutaneous glands, but their purpose is not yet evident. 



