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148 BRITISH PALAEOZOIC FOSSILS. [Crustacea. Ampyx nudus (March. Sp.) Ref. and Syn.—Mem. Geol. Surv. Dec. 2. t. 10. Trinucleus nudus March. Sil. Syst. p. 660. t. 23. f. 5. Sp. Gh.—Entire animal longitudinally ovate, length about one-fourth more than the width; cephalic shield smooth, about two-fifths the length of the animal, about twice as wide as long; glabella very tumid, pyriform, with a short spine projecting from the convex front, narrowed posteriorly and with two short segmental furrows at each side of the base; antennary pores large, in the furrow between the cheeks and the glabella near the front margin ; cheeks longer than wide; thoracic segments six, pleuripedes one-third wider than the axal lobe; pygidium triangular, slightly mucronate, twice as wide as long, the axis with sixteen segmental furrows, the sides with nine. Average length of cephalic shield, five lines. At about half the length of the eye-line there is an elongate swelling which may indicate the position of an eye. Position and Locality.—Very common in the shale at three miles N. of Builth, Radnorshire: Well- field, Builth; Tre Gil, S. of Llandeilo. Genus. OGYGIA (.Brong.) Gen. Char.—Body ovate, flattened, not contractile; buckler semicircular, flattened, with the angles pro duced backwards into flat spines; cheeks large, continuous beneath the front margin; glabella indistinctly clavate, with three very indistinct lateral segmental furrows on each side; eyes semicircular, moderate, cor responding in height to the two anterior lobes of the glabella, reniform ; eye-line coinciding with the margin in the front, then abruptly bending towards the eye, over which each forms the eye-lobe, and thence diverges to cut the posterior margin about the outer third of each side ; thorax of eight segments, axis narrow; pleuri pedes straight, flat, at least twice as wide as the axis, pointed and falciformly bent backwards at the extremities; no facets, a fine mesial pleural furrow not reaching the end ; pygidium semielliptical, with a narrow distinct articulated axis, the lateral lobes also with segmental furrows not reaching the margin, and each having a shorter slightly oblique duplicating furrow. Ogygia Buciii (Brong. Sp.) Syn. and Ref.—Asaphus id. Brongniart. Crust. Foss, and Murch. Sil. Syst. t. 25. f. 2. Sp. Gh.—(Female ?) Broad ovate, one-fourth longer than wide; cephalic shield widely arched, about twice and one-fifth wider than long, as long as the thorax; pleuripedes about twice and one-fifth wider than the axis; pygidium obtusely rounded, the width of the axis and one side lobe taken together at the anterior margin, slightly exceeding its length; axis of about fifteen segments, the sides with thirteen. {Males ?) Narrow ovate, one-third longer than wide; cephalic shield semielliptical, length half the width, as long as the thoracic and one pygidial segments; pleuripedes rather more than twice the width of the axis ; pygidium semielliptical, length slightly exceeding the width of one side, and the axis, about twelve double segmental furrows to the sides, and fifteen to the axis. The figures of Brongniart, Murchison, and Burmeister, represent what 1 here suppose to be the female; the more lengthened elliptical form, with longer head and more pointed pygidium, which I suspect (from the general analogy of recent Crustacea) to be the male of the same species, does not seem to have been previously observed, though nearly as common in all localities as the other* ; with the exception of the more slender contour and difference of proportion indicated above, I see no grounds for thinking there is more than a sexual distinction-—-if others should consider it a distinct species, the name 0. elliptica might be given to it. Position and Locality.—Very abundant in the limestone, shales, and Llandeilo flag, three miles N. of Builth, Pen Cerrig and Wellfield, Builth, Radnorshire ; Llandeilo, Caermarthenshire. * Since the above was written, Mr Salter lias noticed the difference in proportion of different specimens, but without drawing any conclusion therefrom. (See the second Decade of the Geol. Survey.)