PHACOPS. 31 



forward. The basal pair are by far the strongest, and enclose, in conjunction with the 

 interrupted neck-furrow, a pair of transverse, oblong, nearly circumscribed, and very 

 convex basal lobes, not so large as the middle pair. The contrast between the deep basal 

 furrows and the fainter upper ones is striking, and is expressed by Deslongschamps' 

 term " postice bituberculato! n 



The cheeks are regularly convex, the neck-furrow being strongly marked as far as the 

 angle, which seems to have been armed with shorter spines than in P. socialis, Barrande 

 (' Tril. de Boheme/ pi. 26), the species which is most nearly allied to ours. It is not, 

 however, quite certain there were even short spines to the head-angles. 



The outer side of the cheek is moderately arched, and margined by a fainter furrow 

 than the neck-furrow, but continuous with it up to the angle of the head. The axal 

 furrows are not very strong ; and there is no margin in front of the glabella. 



The eyes are small, placed centrally on the cheek, not in advance of this position. 

 The facial suture beneath the eye is nearly direct to the outer margin, and vertical in front 

 of the eye. 



We have not the body. The tail also resembles that of P. socialis ; but has a shorter 

 mucro and fewer ribs. The shape is broad-triangular, a good deal wider than long ; the 

 axis narrow, conical, and rather convex, and at its apex curved upward and passing into a 

 strong, thick, and greatly recurved mucro, whose exact length we do not know, but which 

 was probably as long as the tail itself. There are six flat side-ribs, somewhat arched, and 

 directed obliquely backwards ; they nearly reach the very narrow, flat margin, and are 

 interlined throughout. The sides are tolerably flat, except towards the apex, where they 

 become tumid, and run into the broad base of the thick, recurved spine. 



Comparing P. incertus with the very nearly allied P. socialis, we find the latter with a 

 far more triangular glabella, the furrows of which are more equal ; with spinous head- 

 angles, forward eyes, several more rings upon the axis of the tail, which, besides, has more 

 side-ribs and a much more slender spine — the latter less recurved. But the two species 

 are nearly allied, and the group of trilobites, and shells also, which occur in the subjoined 

 locality, are identical with those of the Lower Silurian sandstone of Normandy, where 

 P. socialis also occurs, and some other Bohemian forms with them. The subject of the 

 geographical distribution of trilobites is worthy of a separate essay. 



Locality. Lower Silurian pebbles, in the " Pebble-bed " of the New Red Sandstone, 

 Budleigh Salterton, South Devon (Mr. Vicary's collection). Also in the "May Sand- 

 stone " of Jurques, Normandy (Eudes Deslongschamps). 



1 Deslongschamps' Latin description is short, and not rery distinctive. " Clypeo triangnlari, angiitis 



brevibus incurvatis fronte magno convexo, antice acuto, postice bituberculato ; genis parvis, oculis 



lateralibus" (p. 317). 



