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Crustacea.] LOWER PALAEOZOIC ARTICULATA. 171 Isotelus (Basilicas) tyrannus (March. Sp.) Ref. and Syn.—Asaphus tyrannus Murch. Sil. Syst. t. 25. f. 1. Sp. Ch.—Cephalic shield semicircular, about twice as wide as long, length equalling the thoracic and three first pygidial segments; glabella obscurely pyriform, convex, not reaching the front margin, with an obscure tubercle on the neck and an oval swelling on each side between the neck and the eye-lobe; eyes with very minute lenses, narrow, crescent-shaped ; thorax, axal as wide as the lateral lobes ; pygidinm elevated, one- third longer than the head, obtusely triangular, one-fifth wider than long, axis less than half the width of the lateral lobes, moderately convex, of about fifteen distinct and many minute segments, sides with about thirteen strong, equal, rounded ribs reaching about two-thirds to the margin, the upper ones reaching nearest to the margin, and nearly at right-angles to the axis, becoming gradually more oblique, and reaching a shorter way as they approach the extremity, leaving a steep, concave, smooth margin, one-third the width of the pleura- all round. Average length of pygidium two to three inches. Position and Locality.—Llandeilo, Caermarthenshire; Craig y Glyn, near Llanarmon Fach, E. of the Benvyns; Rhiwlas, N. of Bala Lake, Merionethshire; Goldengrove, Llandeilo; Nant yr Arian; Tre Gil, S. of Llandeilo. Genus. ILL.ENUS (Dal.) Gen. Char.—Cephalic and caudal shields forming two nearly equal and similar portions of hemispheroids, the articular margin of each being truncated, the base of the glabella and caudal axis only indicated in the respective shields by two short axal sulci; nine or ten body-segments, destitute of pleural grooves, and with long, very narrow, nearly obsolete facets. Eyes and eye-line nearly as in Asaphus. There are three British subgenera:—1st Illamus, having ten body-rings and the lateral cephalic angles rounded. 2nd Bumastus (Murch.), differing only in having no trilobation of the body. 3rd JDysplanus (Burm.), having the lateral cephalic angles produced backwards into spines, and only nine body-rings, otherwise like Illwnus. Subgenus. Illjenus (Dal.) restricted. Gen. Char—Body subcylindrical, with equal rounded ends ; cephalic shield subhemispherical; glabella only defined by two short, parallel, unconnected longitudinal furrows at the posterior margin; eyes smooth, small, semicircular, placed very far apart on the sides of the head; eye-lines subparallel, longitudinal, cutting the posterior margin near the middle of the cheeks, forming the small eye-lobe over the eye and extending thence to two widely-distant points of the anterior margin, where they are connected by a transverse suture; thorax three-lobed, of ten equal segments; pygidium resembling the buckler in size and shape; axis only indicated at the anterior margin by two short parallel unconnected furrows. iLLiENUs Davisii (Salt.) PI. 1. G. fig. 30. Ref.—Mem. Geol. Surv. Dec. 2. t. 2. Sp. Ch.—Head, thorax, and pygidium, of nearly equal length, gibbous; cephalic shield semioval, length rather more than two-thirds the width, very gibbous, the front nearly vertical; axal furrows sigmoid, half the length of the head, slightly nearer in front than at base; the included space about one-fourth wider than the cheeks; eyes rather more than their own length in advance of the posterior margin, and half way between the axal furrows and the margin; thoracic segments with the axal and lateral portions of about equal width, deflexion from the knee close to the axis in the anterior segments, gradually removing to one third of the width in the posterior ones; pygidium semioval, length two-thirds the width, moderately convex, the outer third of the circumference rather abruptly sloping to the margin, axal furrows barely indenting the margin. Surface smooth. Average length of head nine lines. z2