BY .TOHN MITCHELL. 447 



absence of glabellar furrows and lobes; the small convergence of 

 the axial furrows posteriorly, squat condition of the eyes and mild 

 convexity of the eephalon. 



Adastocephalum tbleotypicum, g. et sp.n. 

 (Plate xvi., figs. 1 and 2). 

 Sp. chars. — Cephalon mildly convex, finely granular, and sub- 

 semicircular. Glabella only moderately convex, gently rounded in 

 front, sides rather straight. Neck furrow deep, neck ring moder- 

 ately arched, bases strongly directed forward, and interrupting the 

 continuity of the neck furrow laterally. Limb very narrow. 

 Axial grooves deep, and only converging moderately, posteriorly. 

 Cheeks relatively small, gently convex, genal and palpebral lobes 

 ill-defined; posterior furrows narrow, deep, and with the lateral 

 furrows, which are shallow, form almost a semicircle; posterior 

 ridges narrow, strongly directed anteriorly and merging into the 

 depressed lateral borders in such a way as to obliterate the genal 

 angles. Eyes not prominent, rather squat, and they and the pal- 

 pebral lobes are indistinctly separated from the genal lobes; in 

 th central rows of eye facets there appear to have been four 

 individuals in each row. 



Obs. — The subject of this description is a headshield, of which 

 the left cheek is much damaged, but the rest is fairly perfect, 

 except that the right eye is somewhat distorted. This form is just 

 such as could reasonably have been expected to make its appear- 

 ance before the close of, say, the lower Devonian period, and 

 apparently marks the limit of specialisation in the glabellar of 

 the true Phacops group. Perhaps the nearest relatives of the form 

 now under consideration are Ph. logani Hall, Ph. rana Green, 

 and Ph. hylanderi Clark, for in each of these species the first and 

 second pairs of gabellar furrows are obsolesced. It is also worth 

 noting that our highly specialised type occurs associated with Ph. 

 crossleii and Ph. serratus, which possessed the normal glabellar 

 furrows, though faintly defined. The limb or front lobe of the 

 glabella in each of these two species is obsolete, but in the new 

 form this limb is present, but indistinctly outlined. For purposes 



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