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Crustacea.] LOWER PALAEOZOIC ARTICULATA. 169 Genus. ASAPHUS (in a wider sense than Brong.) Gen. Char.—Head and tail nearly equal; eyes with a firmly fixed, thick cornea, with a smooth external surface ; facial suture cutting the posterior margin of the cephalic shield within the lateral angles; thorax with eight to ten segments, having large facets and distinct, wide, nearly straight pleural grooves, not reaching the margin ; globelia indistinctly defined in front; pygidium with the segments usually indistinctly marked, the axis generally distinct and annulated, when traceable, always elongate conic (of the ordinary type). Contains among others the following subgenera, 1st, Asaphus (not British, taking the A. cornigerus as the type) ; 2nd, Isotelus. Subgenus. ISOTELUS (Behai/). Gen. Char.—Large, elliptical; buckler semielliptical, with the angles rounded, or produced backwards into spines; glabella indistinctly defined ; eye-lines meeting at an acute angle at the front margin, thence diverging backwards, slightly approximating again about the middle of their length, where the “hiant” eyes are situated, and again diverging to cut the posterior margin near the angles; thorax of eight segments, axis as wide or wider than the lateral lobe, the ends of the pleurae rounded, with a strongly marked triangular facet and pleural groove; pygidium, resembling the buckler in size and shape, trilobed, and generally with a broad smooth mar gin ; axis and lateral lobes with fine segmental furrows, or smooth. There seem to be two types, 1st, the Isotelus (gigas, &c.), having the angles at the head rounded, and no distinct segmental divisions of the pygidium ; 2nd, (Asaphus tyrannus, &c.) having the lateral cephalic angles produced into spines, and both the axis and lateral lobes of the pygidium distinctly defined and divided into simple segments. After a careful review of all the species I do not, however, feel competent to separate them; if it should be found convenient to distinguish them as subgenera, the name Isotelus should be retained for the former, and the latter might be called Basilicus, as suggested by Mr Salter, in the 2nd Decade of the Geol. Survey. Basi- licus differs from Ogygia, which it closely resembles in shape, in the facets of the pleura, as well as the simple (not duplex) furrows of the pygidium; the glabella is more prominent than in Isotelus proper. Isotelus affinis (M c Coy). PL 1. F. fig. 3. Sun. and Ref.—Isot. qiqas, I. planus and I. Powisii of Portk. Geol. Rep. (not of the writers he refers to) PI. 6. f. 1. and t. 7. f. 2 and 3. Sp. Ch.—Axis of the body only slightly exceeding the pleurae in width; pleurae gently arched downwards at a very obtuse angle; from about half way between the axis and the extremity, a large pleural furrow reaches from the axis to about one-third of the truncated extremity of each; pygidium flattened, semielliptical, or slightly trigonal, from the straightness of the sides; axis narrow, sharply defined, gently convex, reaching as far as the concave space round the margin. In general proportion this resembles the Isotelus gigas (Dekay), from all the varieties of which it is dis tinguished when specimens of the same size are compared, by the much greater flatness or depression of all its parts, the long, narrow, sharply defined axal lobe of the pygidium, and the much greater length of the pleural groove of the pleurae (nearly double that of the I. gigas), and distance of knee from the axis, as well as their slight degree of deflection (being bent at nearly right angles at one-third from the axis in I. gigas). The pygidium differs from that of the I. Powisii (Murch. Sp.) by the absence of all segmental furrows on the lateral lobes, except the first, and by the more pointed outline and narrow margin. Position and Locality.—Not uncommon in a schist over the iron-works at Tremadoc, Merionethshire; very similar in appearance to that at Pomeroy, county Tyrone, which afforded the species to Gol. Portlock. jExplanation of Figures.—PI. 1. F. fig. 3. Pygidium and thorax natural size, from the fine slates over the iron-works of Tremadoc.