50 TRILOBITES OF GIEVAN. 



Genus STYGINA, Salter. 

 1. Stygina latifrons (Pprtlock), 1843. Plate VII, fig. 10. 



1843. Asaplms latifrons, Portloek, Geol. Bep. Londond., p. 292, pi. vii, figs. 5, 6. 



— Asaphus marginatus, Portloek, ibid., p. 293, fig. 7. 



1853. Sfijglna latifrons, Salter, Rep. Brit. Assoc, Trans. Sect., p. 59 (read 1852). 



1854. Stygina latifrons, Morris, Cat. Brit. Toss., 2nd ed., p. 115. 



1859. Stygina latifrons, Salter, in Murchison's Siluria, 2nd ed., p. 184, Foss. 26, fig. 2. 



1864. Stygina latifrons, Salter, Mem. Geol. Surv., dec. xi, pi. ii. 



1866. Stygina latifrons, Salter, Mon. Brit. Trilob., p. 172, pi. xviii, figs. 7—10. 



1869. Stygina latifrons, Linnarsson, Vestergotl. Camb. Silur. Aflagr., p. 77, pi. ii, figs. 41, 42 



(Kongl. Svensk. Vet. Akad. Handl., vol. viii, No. 2). 

 1877. Stygina latifrons, Woodward, Cat. Brit. Foss. Crust., p. 59. 

 ? 1899. Stygina latifrons, Mem. Geol. Surv., Silur. Eocks Brit., vol. i, Scotland, pp. 513, 674, 689. 

 1900. Stygina latifrons, Wiman, Bull. Geol. Instit, Upsala, vol. v, pt. 2, No. 10, p. 171, pi. v, 

 figs. 16—18, ?19; pi. vii, fig. 17. 



Specific Characters. — General form depressed, elliptical. 



Head-shield as long as the pygidium, more than one third the whole length, 

 nearly a true semi-oval, evenly convex (except on the posterior median portion, 

 which is somewhat abruptly raised), sloping on all sides to a concave border. 

 Glabella pyriform, scarcely defined in front, much contracted posteriorly but 

 expanding suddenly upon neck border; greatest width not more than one fifth 

 that of the head. Eyes small, convex, much curved, situated at less than their 

 own length from the hinder margin opposite the contracted part of the glabella, 

 and rather further apart than the width of the thoracic axis. Facial sutures 

 bend outwards behind the eyes to meet the posterior margin at a very acnte 

 angle ; the anterior branches describe a large arc, diverging from the eyes at an 

 angle of 70° and cutting the anterior margin far out in a line with the fulcral 

 points of the thorax. Free cheeks triangular; genal angles produced into short 

 points. Occipital segment obscurely marked. 



Thorax of nine rings, not so long as the head. Axis convex, cylindrical, about 

 two thirds the width of the pleura?. Pleurae flat as far out as the fulcrum, not 

 grooved. Fulcrum situated at about two thirds the length of the pleurae. 

 Extra-fnlcral portion of pleurae bent down and slightly backwards and facetted for 

 rolling up. 



Pygidimn semi-oval, blunt, not convex. Axis conical, with rounded obtuse end, 

 about half as wide as the lateral lobes and quite two thirds as long as the pygidium, 

 with about eight faint rings, very indistinct posteriorly. Lateral lobes with inner 

 portion gently convex, outer portion broadly concave with someAvhat sharply 



