68 BRITISH FOSSILS. 



Our figure shows it compressed a little obliquely, but, allowing 

 for this, it is not far from the true shape, and it is probable that 

 not much of the margin is lost on the right-hand side, while enough 

 remains of the other to indicate that the outline was somewhat 

 curved. Two other specimens in Lord Kinnaird's cabinet show the 

 curved outline. The eyes are distinctly visible on each side, and are 

 as large as crown pieces. 



The head must have been considerably more convex than at 

 present, since in our flattened specimen it is corrugated all over ; it 

 is sub-pyramidal in shape, truncate or but slightly arched in front 

 between the eyes, and concave posteriorly. The side margins (so far 

 as visible) and the front are strongly crenate, but not tubercular ; 

 the general surface appears smooth or minutely punctate, but not 

 anywhere marked by plicae. The eyes occupy the outer angles of 

 the head ; they are oval, an inch and a half in the largest diameter, 

 and about an inch broad. The lenses are rather large, about eight 

 rows in one-tenth of an inch, and in this pressed specimen are 

 rhomboidal rather than hexagonal, at least in arrangement ; this 

 may be due to pressure only. 



The entire head, compared with that of P. acuminatus, is broad 

 and short, and much narrowed in front, so as to be intermediate in 

 shape between that square-headed species and the forms with 

 semioval carapaces. The eye is much larger in proportion. 



Epistoma and Lalrum, Plate III. figs. 2-5 (6 ?). This piece, 

 which is the epistoma or under side of the front of the head, occurs 

 in tolerable plenty, and is the portion formerly supposed to be the 

 carapace itself (see p. 6). From a very perfect and large specimen in 

 the British Museum, a reduced figure (fig. 5) is added, which justifies 

 the inference above drawn as to the great size of the animal. The 

 figured specimen, 2, is the same as that figured by Agassiz, and is 

 nearly three inches from back to front, but we have other specimens 

 which measure four inches and a half long, and were probably 

 -sixteen or eighteen inches wide. 



The " seraphim," as the piece is called, is straight or only sinuous 

 in front, but broadly wedge-shaped behind ; its extremities are un- 

 known, unless fig. G be the lateral termination on one side. It 

 consists of a central arrow-shaped piece and two widely-expanded 

 wings or lateral lobes which project backwards nearly as far as the 

 shaft or central lobe, forming with it the broad salient angle of the 

 posterior margin. These lobes are not, however, united with the 

 shaft but overlap it with thickened edges on each side, giving it an 

 appearance of contraction in the middle ; it is, when detached, a 



