THE ANCIENT ROCKS OP ST. DAVID'S. 187 



suddenly to a rather sharp point, the lobes marked with four raised 

 ribs, with their points extending beyond the margin. 



The small size, the position of the fulcral point of the pleurae, and 

 the deeply pitted cheeks distinguish this species from P. Zippei, to 

 which otherwise it is most nearly allied. 



Locality and Formation. Llanvirn Quarry (near Aber-eiddy) : 

 Upper Arenig. 



Phacops llanvienensis, n. sp. PI. IX. figs. 3, 4. 



This species and Phacops Nicholsoni, Salter, from the Upper 

 Skiddaw slates of Cumberland, are the earliest species of the genus 

 known in this country. This species was found in the Llanvirn 

 quarry, and therefore in beds of equivalent age to the Upper Skiddaw 

 slates. 



Description. — Oval in shape, rather more than 2 inches long by 

 1J inch wide. Head one third of the whole length, semicir- 

 cular, with the angles rounded. Glabella expanded in front and 

 equal to one half the whole width of the head, but narrowing 

 rapidly towards the neck until it is reduced to less than half the width. 

 The neck-furrow is deep ; and the basal furrows indent the sides 

 to about a third of the distance across ; the remaining furrows, of 

 which apparently there are three on each side, are rather indistinctly 

 marked. The cheeks are triangular, with a strong posterior margi- 

 nal furrow; surface gently raised and smooth. The eyes small, 

 situated near the anterior margin and close to the glabella. Thorax 

 of eleven moderately convex rings ; axis narrower than the pleurae, 

 widest at about the fourth or fifth segment ; pleurae deeply grooved 

 to three fourths of their length. Tail semicircular, arched in front, 

 moderately convex and with a tolerably distinct margin; axis 

 conical, reaching nearly to the posterior margin and marked with 

 eight or nine rings ; the lateral lobes show four or five distinct ribs. 

 This species differs in many points from any other British species, 

 and can easily be distinguished by its wide and only slightly 

 inflated glabella, and its small eyes situated far forward. 



Locality and Formation. Llanvirn Quarry : Upper Arenig. 



Calymene Hopkinsoni, n. sp. PI. X. figs. 4, 5. 



A small species, and one of the earliest of the genus known. Found 

 by me in the Llanvirn quarry some years ago. I have named it 

 after my friend Mr. Hopkinson. 



Description. — 1| inch long and about f of an inch wide. 



Glabella in length and width about eqnal, sides nearly straight, 

 with three well-marked lateral furrows directed obliquely forward ; 

 apex flattened. 



The axis of the thorax convex, less than a third of the whole 

 width, and tapering regularly towards the tail. 



Pleurae flat and deeply grooved to about two thirds of their length. 



The tail strongly margined, axis of eight or nine rings, and lobes 

 marked with five or six ribs. 



