2 



BRITISH FOSSILS. 



I Lave figured this fine species from General Portlock's original 

 specimens, and I follow most writers in adopting De Kay's name, 

 because it must have been contemporary with the publication of 

 Stokes' paper (though the latter was read early in 1823). Even 

 were A. platycephalus a little the earlier name, it was published 

 without any description. And ifc is just possible that the fossil 

 described by Stokes may belong to a different species. 



Description. — General shape oval-oblong, with the sides rather 

 straight, the head and tail nearly equal, and both subtriangular, 

 the head pointed, the tail more obtuse at the tip. The surface is 

 convex when the fossil has not undergone compression, a line taken 

 from the snout to the apex of the tail being a regularly convex one, 

 uninterrupted by any neck furrow, depression, or convexity of the 

 smooth and even bod}^ rings, or furrows on the axis of the tail. The 

 axal furrows are very obscure in the head ; they are neatly marked 

 but shallow along the body, and only very faint along the tail. 

 All the surface is smooth. The sides are strongly deflected, but not 

 Kiteep. 



The head has the shape of a broad and pointed Gothic arch, the 

 breadth at base being to the length as three to two. The margin is 

 very narrow and flat, rather than recurved. The facial suture forms 

 a broad ogive arch in front, running for some distance close within 

 and parallekto the front margin ; and, beneath the eyes, which are 

 large, placed near the glabella, and rather behind the middle of the 

 head, the suture curves gently out and cuts the posterior margin mid- 

 way from the axal furrovf. The hinder angles are blunt pointed, 

 not rounded. On the under side of the cheek near the point is a 

 convex space, containing an oval depression, which receives the 

 apices of the front pleurae in rolling up (fig. 6, and see also fig. 5 

 for the cast of this depression on the matrix). The labrum (fig. 7) 

 has a narrow base, then a strong constriction, and thence the sides 

 are parallel. The apex is deeply furcate, the parallel forks occupying 

 nearly half the entire length of the organ. Body rings smooth, 

 rounded at the apices, deflexed at the fulcrum, which is placed rather 

 beyond one-third, and with a broad strong groove. Tail sub- 

 trigonal, with straight sides, and rounded blunt tip. The faint axis 

 rapidly tapering, broad conical, and reaching three-quartei's the 

 length. Sides quite si.iooth. 



In young specimens, says Hall, the caudal extremity is more 

 pointed, and exhibits marks of eight articulations ; in older spe- 

 cimens these increase in number. But the crust presents many 



