2 



BRITISH FOSSILS. 



Description, — Length of adult about three-quarters of au inch. 

 Form broadly oval, tapering backwards. Head large, semi-lunar, 

 slightly but regularly convex. The central shield, composed of 

 glabella and fixed cheeks, is widely semicircular, emarginate exter- 

 nally, with a contour nearly corresponding with that of the head. 

 The head is bordered by a distinct but not very broad, smooth, 

 sliglitly concave margin, which is continuous with the central 

 shield through the anterior half of the margin, but is cut av/ay at 

 the exterior fourth oii each side by the curving inwards of the 

 facial suture. The glabella is rounded-trigonal, in front trans- 

 versely ova] and very convex, and slightly prolonged and contracted 

 behind, where it is much flattened ; the crust is perfectty smooth. 

 The two posterior glabella furrows on either side are represented 

 by shallow pits, arranged nearly in a square, and occupying the 

 posterior third of the glabella; the anterior pair of furrows by 

 similar depressions more remote, placed near the anterior and outer 

 angles of the middle third. Two longitudinal grooves connect the 

 posterior glabellar depressions with the neck furrow, only shown in 

 our upper figure. The tergal portion of the neck segment is small 

 and convex. The fixed cheeks are large, slightly arched, coalescing 

 before the glabella in a narrow ridge, bordered by a still narrower 

 margin. The lateral portion of the occipital groove passes forwards 

 and outwards two-thirds across the fixed cheek, then slightly back- 

 wards to the lateral margin, cutting off* nearly one-third of the 

 cheek. The portion of the cheek before the groove is smooth like 

 the glabella, the portion beliind it (the neck segment) is marked 

 with delicate strise parallel to the furrov/ and to the posterior 

 margin. 



In all our examples the free cheeks and the eyes are absent, 

 but from the portions which we possess, and from the analogy 

 of closely allied forms, we may safely supply a facial suture 

 coinciding with the outer edge of the anterior margin, appearing 

 upon the upper surface nearly midway between the centre of the 

 frontal edge and the genal angle, passing gently inwards, and then 

 outwards and backwards through the limb, slightly emarginating 

 the semicircular contour of the head, and so curving downwards 

 and outwards towards the genal angle. 



The linear free cheek, bearing probably a linear eye, is absent. 

 In our specimens the posterior angles have an imperfect truncated 

 look, and we should be inclined to believe that, following the 

 analogy of Trinvcleus, Lionide, Cyithoniscus, &c., the lower edge 



