AGNOSTUS NUDUS. 15 



Grciinvall has described and figured the tail only of this species, but thinks it 

 not impossible that some of the heads which he has referred to A. luudgreni may 

 really belong to A. rotundus. According to him the tail of A. lundgreni var. nana 

 is the nearest to the tail of A. rotundus, but is nevertheless distinctly different. 

 In A. lundgreni var. nana, it may be observed, the axis is distinctly segmented, 

 and the lateral lobes are separated by a furrow behind the axis. 



Our own specimens are so imperfect as regards the head that the description 

 here given may well be incorrect. It seems, however, that even in perfect 

 specimens the glabella cannot be very distinct. 



Horizon and Locality. — Menevian : Porth-y-rhaw, St. David's. 



11. Agnostus nudus (Beyrich). Plate II, fig. 5. 



1845. Battus nudus, Beyrich, Ueber einige bahmische Trilobiten, p. 46, fig. 20. 



1846. Battus nudus, Barrande, Notice preliminaire, p. 15. 



1847. Phalacroma nudum, P. ^marginatum, P. carinatwm, P. scutiforme, P. gibbosum, Hawle and 

 Corda, Prodrom., pp. 43-45, pi. iii, fig. 20. 



1852. Agnostus nudus, Barrande, Systeme silurien, vol. i, p. 903, pi. xlix. 



1879. Agnostus nudus, var. marginata, Brogger, Nyt Mag. for Naturv., vol. xxiv, p. 73, pi. vi, fig. 3. 



1880. Agnostus nudus, var. scanicus, Tullberg, Agnostus-artema, p. 29, pi. ii, fig. 18 a, b. 

 1880. Agnostus nudus, var. marginatus, Tullberg, ibid., p. 29, pi. ii, fig. 19 a — d. 



1895. Agnostus nudus, Pompeckj, Jahrb. k. k. geol. Eeiclis., vol. xlv, p. 518, pi. xvi, figs. 7 a, /<, c, 

 8a,b. 



Head smooth, forming rather more than half an ellipse, with a very narrow 

 margin (invisible in the British specimens, owing no doubt to imperfect preserva- 

 tion), and without any trace of axial furrows. 



Tail smooth, forming rather more than half an ellipse, with a broad margin, 

 which is widest posteriorly and narrows rapidly towards the sides. The central 

 portion, corresponding to the axis and lateral lobes of most Agnosti, is nearly circular 

 in outline, is separated from the margin by a well-defined groove, and bears a small 

 median tubercle near its front border. There is no trace of axial furrows. 



Head- and tail-shields about 4 - 5 mm. long, 4 mm. wide (in a somewhat distorted 

 specimen). 



I have seen only one British specimen which can be referred with certainty to 

 this species, and it appears to belong to the form which has been distinguished by 

 Brogger as A. nudus var. marginatus. 



A. barloivi and A. eskriggci are the only British species with which it is likely 

 to be confounded. In both these forms the base of the glabella is defined by two 

 very short axial furrows, and in J. barlowithe axis of the tail is similarly indicated. 

 In A. nudus, on the other hand, there is no trace of differentiation of the axis on 



