PHACOPS. 21 



P. (Phacops) Stokesii, Milne-Edw. PL II, fig. 1 — 6. 



Asaphus macrophthalmus, Brongniart. Crustaces Fossiles, pi. i, fig. 5 {not fig. 4), 



1822. 

 Calymene macrophthalma, Buckland. Bridgw. Treatise, pi. lxiv, fig. 4 {not fig. 5), 



1836. Murchison, Silur. System, pi. xiv, fig. 2, 1837. 



— Stokesii, Milne-Edwards. Crustaces, 3, 324, 1840. 

 Portlockia suBLiEVis, M'Coy. Sil. Foss. Ireland, pi. iv, fig. 13, 1846. 



Phacops Stokesii, Salter. Memoirs Geol. Survey, vol. ii, pt. 1, pi. 5, fig. 1, 1848. 

 Quart. Geol. Journal, vol. vii, pi. ix, fig. 2, 1850. 



— — M'Coy. Synopsis Woodw. Mus., p. 163, 1851. 

 — Salter. Siluria, 2nd edit., pi. 18, fig. 6, 1859. 



P. {Phacops) 1 — 2 uncias longus, ovatus, granulatus. Glabella parum inJlata,frontem 

 impendens, sphcerico-trigona, lobis basalibus rotundatis, reliquis omnino {etiam intus) 

 obscuris. Oculi magni, insuper depressi. Thorax axe angusto. Cauda brevis semicircular is, 

 convexiuscula, immarginata, axi depresso b-annulato, lateribus 4 — b-sulcatis, sulcis abbrevi- 

 atis obscuris. 



This is a very common Upper and Middle Silurian fossil, but is never found in Lower 

 Silurian Rocks. Yet it is but imperfectly known, being generally in fragments. And it 

 has been moreover a troublesome one to the palaeontologist, owing to the original mistake 

 of Brongniart, in confounding it with the true tnacrophthalma. And, until lately, specimens 

 of the more common P. Musheni, our next figured species, were constantly mistaken for 

 it. I believe the above synonyms may all be trusted, and I do not cite the more doubtful 

 ones. 



The species is rather a small one, seldom exceeding an inch or an inch and a half in 

 length, and is obtusely ovate and very convex. The head occupies less than a third and 

 more than a fourth of the whole length, and is semicircular, and wider than the thorax ; the 

 glabella is an equilateral spherical triangle, overhanging the front and the eyes, and much 

 narrowed behind, where the basal lobes are distinct enough as small tubercles on each 

 side of a linear lobe. This is overhung by the inflated upper lobes, which are confluent, 

 and show only very obscure traces of any of the upper furrows, even on the inner cast, 

 where they are strongest. The median furrows are short and curved, the upper strongly 

 bent midway, as in many species of the subgenus Phacops. Surface of glabella covered 

 with a fine granulation. 



The cheeks are triangular, with a strong, continuous, marginal furrow. The eyes large, 

 much depressed above, and with the lentiferous surface gently curved, of numerous 

 lenses, and so placed that the forward end nearly touches the glabella, while the base is 

 widely distant, and does not moreover reach the sharp neck-furrow. 



The body has a convex axis, narrower than the pleurae, which have a deep sharp groove 

 for two thirds their length, and are steeply curved down beyond the fulcrum, the latter 

 being placed less than half-way out in the front rings, and at one third behind. 



