THE A^CIEXT ROCKS OF ST. DAVID'S. 183 



Description. — Length, nearly an inch, and a qnarter, width, rather 

 less than the length. In shape ovoid, with the head-spines reaching 

 directly backwards to a little beyond the tail. The head occupies 

 about half the length, and, excluding the spines, is semicircular in 

 shape. The fringe-border is nearly equally wide throughout, and 

 has three or four rows of puncta. The glabella, which reaches the 

 inner edge of the fringe, but scarcely indents it, is strongly convex 

 and wide anteriorly, and tapers towards the neck-fnrrow ; the sides 

 are indented by two pairs of furrows, converging in the centre. 

 The cheeks are moderately raised and like the glabella, and have the 

 whole snrface finely reticulate. The body has six rings, with a 

 strongly raised tapering axis occupying about a fourth of the width. 

 The tail is sharply triangular, and has a very broad margin, which 

 is equally deep with the lobes at its widest part. Axis of five or six 

 rings, and lateral lobes deeply ribbed. 



This can scarcely be confounded with any other British species ; 

 the fimbriated border, like that of T. Murchisoni, has interposed a few 

 short rays, which may tend to mislead one if the other characters are 

 not looked into. It is, however, a much larger and wider form than 

 T. Murchisoni, the tail has a stronger and wider margin, and the 

 lobes are more distinctly ribbed. 



Locality and Formation. Llanvirn quarry : Upper Arenig. 



Trixfcleus Ramsayi, n. sp. PI. X. figs. 1, 2, 



This species was found in the black shale at Porth-hayog, in 

 Ramsey Island, in the summer of 1872 ; and specimens of it have 

 since then been found in the black slates between the Ash beds in 

 Abereiddy Bay. I have much pleasure in naming it after Prof. 

 Ramsay, in whose company it was found, and to whom we owe so 

 much for his very valuable researches amongst these ancient rocks 

 in Xorth Wales. 



Description. — Head semicircular, f of an inch long by -| of an 

 inch broad ; fringe-border equally wide throughout, except where 

 indented by a convex ridge in front of the glabella ; it has five or 

 six rows of closely set puncta, and the extreme outer margin has on 

 it a row of moderately strong spines. Glabella occupying about one 

 fourth of the width of the head, strongly convex and pyriform in 

 shape, and indented by two pairs of strong furrows behind. Cheeks 

 moderately convex, and the whole surface covered over with minute 

 puncta. The axis of the thorax is narrow and convex. The tail 

 has not been found. 



This is a very well-marked species, and differs from T. Lloydii, 

 to which it is most nearly allied, in having a more evenly rounded 

 glabella with lateral furrows, the surface of the cheeks covered 

 over with minute puncta, the outer margin of the head covered 

 with minute spines, and the angles scarcely at all expanded. 



Locality and Formation. Porth-hayog in Ramsey Island, and 

 Aber-eiddy Bay : Upper Arenig and Lower Llandeilo groups. 



