ASAPHUS. 155 



nothing to do with the form described by Portlock under the name Isofelus Powisii in 

 1843 ; nor is it easy to see why Prof. M'Coy retained it under that subgenus, since 

 the facial suture is exactly that of Basilicus. It is one of the commonest of Caradoc 

 fossils. From A. tyrannus the rounded shape, faint furrows on the pygidium, and quite 

 different head, without any furrows to the glabella, easily distinguish it. 



Rarely six inches long, but still a large species, of truly elliptic, very regularly convex 

 form ; the semilunar head much shorter than the thorax, and this shorter than the tail. 

 The glabella is only convex and rounded in front ; behind, it neither shows convexity, 

 separation from the cheeks, nor lobes of any kind. The eyes are rather large and lunate, 

 but not elevated, and, measured from their outside, are wider apart than the length of 

 the head ; they are not at all sunk in the free cheeks, and their curved lentiferous suiface 

 contains about 7000 lenses, according to observations of mine in 1849 (Mus. P. Geology, 

 and Collection of the late Daniel Sharpe). The cheeks are very little convex, without 

 any marginal furrows, and produced into very short head-spines. 



Thorax distinctly three-lobed, the axis broad, wider than the pleurae, but not strongly 

 separated from them ; a punctum in the cast marks the point of junction. The pleural 

 groove is bordered, as in A. tyrannus, by strong ridges, meeting beyond it in a sort of 

 node ; and it extends as a broad, well-defined furrow for more than half the length of the 

 pleurae. These have the fulcrum near the axis, not above one third out, and at nearly 

 an equal distance in all the rings. Prom the fulcral point the pleurae bend down, but 

 not backward, and are somewhat recurved at the tip. 



The tail is the truly characteristic portion, and has a whimsical resemblance to that of 

 the Homalonotm bisulcatus (PI. X, fig. 3), with which it is so often found in company. 

 It is semioval, rounded at the end, and regularly convex, the axis not greatly raised 

 above the general convexity ; broad at its base, where it occupies rather more than one 

 third the whole width of the tail, but soon narrowing behind, and becoming more parallel- 

 sided. It reaches quite to the inner edge of the concave margin, fully five sixths down 

 the tail, and is there rounded and a little prominent, especially in young specimens 

 (figs. 3 and 4). These indeed have the axis generally more strongly marked out than in 

 the adult. The axis is marked by about eight or nine obscure ribs, and the sides by 

 an equal number of rather short and obscure furrows ; but the upper or border furrow, 

 always the deepest in the AsapJdda, is here distinctly and strongly so, and hence, together 

 with the wide pyramidal axis, the resemblance to the Homalonotus aforesaid. The limb 

 is concave all round, and the caudal fascia, rarely seen, is only slightly indented by the 

 tail-axis. 



In all the above characters, — the absence of glabella-lobes, the smooth and almost 

 unfurrowed tail, with its broader and more pyramidal axis, &c., the species is distinguished 

 from A. tyrannus. The labrum (fig. 6) is also different ; longer, more indented on the 

 sides, less raised in the centre, with very obhque lateral tubercles and shorter forks to 

 the apex. 



