132 SILURIAN TRILOBITES. 



Tail semicircular, but with rather straight sides and a broad, coarsely striate margin, 

 which is waved by the strong lateral furrows. The axis narrows abruptly from a conical 

 base, and is then parallel-sided ; reaching nearly to the end of the tail. It is 9-ringed, 

 and has, besides, a blunt terminal portion, and no appendix beyond. The sides have 

 eight furrows, strong and deep, and rounded at their ends, reaching to the striated 

 border, but not the margin ; the upper or marginal one much the strongest and deepest. 

 The ribs are rounded, very convex, quite straight, radiating, and without any duplications. 

 In the tail of young specimens, however, a faint duplication is observable in the forward 

 ribs, and only on the forward edges of these ribs, making them angular instead of 

 rounded. The whole surface of the tail is minutely granular ; of the glabella, apparently 

 punctate.^ 



Variations. — There seem to be the usual "forme longiie" and ''forme large' in this 

 as in other species of Ogygia. Our principal specimens (figs. 9, 11) are of the broad 

 form, and the smaller ones (figs. 1, 7) of the narrower one. Figs. 1 and 7, which are 

 probably $ forms, have the head much more pointed, forming a gothic arch, and the 

 margin in front of the glabella much broader than in the female form ; the glabella is 

 more pointed and clavate in front. The thorax, too, is relatively narrower, and the tail 

 a semicircle, but with somewhat straight sides. Young specimens do not differ much 

 in proportion or shape from older ones, except in having the tail more triangular (fig. 7), 

 and with a narrower axis, the number of ribs in the tail remaining the same. But very 

 young ones (figs. 5, G) show the metamorphosis. Fig. 5, which is only two lines long, 

 has clearly but six rings, and fig. 6 has only seven rings, besides having the cheeks 

 contracted at the base of the spine, which is also more divergent than in the older 

 form. This last character may be accidental in this specimen, for somewhat older ones 

 (figs. 3, 3, 4) show the spines more parallel to the body, and the cheek not contracted 

 at this part. 



The appearance of a reduced number of body-rings is often due to accident. In fig. 

 4 the body has slipped under the carapace, so that on the left side there are seven rings, 

 and on the right hand only four. But this is not the case in the two youngest specimens 

 just quoted, which have six and seven rings respectively. 



Locality. — Llandeilo Flags, Gilwern, near Llandrindod, Builth. All the specimens 

 figured are in the collection of Mr. Griffith Davies, of Islington, except fig. 8, which is 

 Murchison's original specimen from the Corndon Mountain W. side (Mus. Geol. Soc.) 



^ A delicate lincation occupies the whole of the axal-lobe of the thorax, aud the outer half of the 

 pleurae is much more strongly marked with it than the inner. The concentric lineation of the tail-border 

 is very sharply defined, in a rather narrow band, the lines abutting obliquely, as usual, against the inner 

 edge. 



