BRITISH FOSSILS. 



3 



suture cuts the outer margin in a curved line in front of the pos- 

 terior angles, and opposite the base of the eye ; on the under surface 

 of the head (fig. 5) the suture cuts the margin further back- 

 ward (b b). Above the eye it continues along the axal furrow and 

 round the front of the glabella just outside the marginal furrow. 

 Eyes rather large, conical, rising in some specimens nearly to the 

 level of the glabella, placed about half-way up the cheek, near to 

 the two upper glabella lobes, and occupying their length : eye lobe 

 with a raised outer margin ; lentiferous surface broad, with about 

 155 lenses in each eye, each vertical row containing eight. The 

 cornea is convex over the lenses, and the intermediate flattened 

 spaces are finely granular, the granules forming a roagh hexagonal 

 network toward the base of the eye ; the lenses are nearly their own 

 diameter apart, but this varies much in different individuals, the 

 space being often much less (figs. 7, 8). 



On the under side of the head, the incurved front portion (which, 

 as in all the genus, is continuous across,) is broad (fig. 5, a), and 

 granular, like the upper surface ; it supports the broad base of the 

 hypostome, which is also granulated. Tliis organ is subquadrate 

 but broadest at its base, and very regularly convex, almost tumid ; 

 a faint concentric furrow running round the sides and tip just indi- 

 cates a narrow margin, more flattened than the other parts ; there 

 are no lateral furrows, but high up on each side is a small tubercle. 

 The tip is straight and somewhat truncate, and the exterior angles 

 are cut ofl* so as to render the end somewhat polygonal ; but tliere 

 ;are no traces of projecting teeth, and the appearance of the apex is 

 obtuse. The entire organ is much narrower than the glabella, and 

 not above half its length, but from the position of its base it reaches 

 as far backward as the middle pair of glabellar furrows. And these 

 glabellar furrows, as Burmeister has shown, doubtless indicating the 

 position of the jaws and accessory parts of the mouth, the hypostome 

 must have served the ofiice of labrum or upper lip. 



Thorax considerably longer than the head, of 1 1 not very highly 

 arched rings — the axis moderately convex, of nearly equal breadth 

 with the pleurae. These, which are traversed by a straight deep groove, 

 (fig. 10, d), are curved rather abruptly downwards at the fulcrum 

 (fig. 10, e), which anteriorly occurs at the inner third of their length, 

 and in the posterior ring does not reach further than one fourth. 

 The anterior edge of each pleura is sharpened or facetted * to pass 

 under the preceding one, and the posterior edge is thickened. Each 

 pleura is much bent forward at its end, which is deeply notched 



* M'Coy, Annals Nat. Hist, (Dec. 1849.) 



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