80 TRILOBITES OF GIRVAN. 



1885. Phaetonides stokesi, Lindstrom, Forteckn. G-otl. Silur. Crust. (Ofv. K. vet. Akad. Forhandl., 



No. 6), p. 75. 

 1899. Proetus stokesii, Mem. Geol. Surv., Silur. Bocks Brit., vol. i, Scotland, pp. 550, 673. 



Specific Characters. — General shape longitudinally oval. 



Head-shield semi-elliptical to parabolic, with genal angles produced backwards 

 as broad flattened pointed spines, closely pressed to sides of thorax and reaching 

 as far as pygidium; broad rounded tumid border surrounding head-shield and 

 extending to tip of spines. Glabella semi-oval, subcorneal, convex, about three 

 fourths the length of the head and one third its width, slightly expanded posteriorly 

 at sides, broadest at base, truncate behind, rounded in front, narrowing anteriorly, 

 and not reaching marginal furrow in front. Pair of large subtriangular basal 

 lobes present, about half the length of the glabella and less than one third its basal 

 width, projecting somewhat laterally, and marked off by strong slightly curved 

 oblique furrows, inclined at about 60° to occipital furrow, and arising a little behind 

 front end of eyes. (The anterior portion of these furrows is in some specimens weak 

 or almost obsolete.) No other lateral furrows present. Occipital furrow strong. 

 Occipital ring broad, of uniform width, with pair of well-defined oval occipital lobes. 

 Fixed cheeks very small and narrow, but expanding in front of eyes and uniting 

 as pre-glabellar band in front of glabella. Facial sutures curving inwards strongly 

 from anterior margin to eyes, and behind them running vertically back about half- 

 way towards posterior margin and then bending out sharply to cut it at an acute 

 angle below or a little outside eyes. Free cheeks large, long, bent down on each 

 side, produced posteriorly into broad flattened genal spines, overlapping at base the 

 extremities of the first and second thoracic pleurae and reaching back to pygidium. 

 Byes reniform, large, convex, prominent, closely pressed against glabella and 

 extending about half its length, situated at about two thirds their own length 

 from hinder margin of head-shield. Neck furrow strong. Neck segment 

 rounded, narrower than occipital ring, but equal in width to cephalic border and 

 continued to tip of genal spine. 



Thorax of ten segments ; axis convex, subcylindrical, tapering gently to 

 pygidium ; at front end equal in width to pleura3, but considerably less wide 

 posteriorly. Axial rings without lateral nodules or only with very faint traces of 

 them. Pleurae gently curved down, with median, scarcely diagonal furrow ; 

 extremities pointed, recurved, and slightly falcate. Fulcrum strong, situated at 

 about one third the length of pleurae. 



Pygidium semi-elliptical to semicircular, with broad tumid border. Axis conical, 

 convex, prominent, tapering slightly posteriorly to rounded tip reaching border; 

 less than one third the width of pygidium at front end ; annulated with 8 — 10 

 rings. Lateral lobes arched down slightly, and marked with about six radiating 

 flattened pleurae, each with fine median furrow. Margin simple, entire. 



