172 SILURIAN TRILOBITES. 



Stygina latifhons, PortlocL PL XVIII, figs. 7 — 10. 



AsAPHUS LATiFBONS, Portlock. Gcol. Rep. Londonderry and Tyrone, pi. vii, figs. 5, 6. 



— M.\RGiNATUs, Portlock. lb., fig. 7, 1843. 



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



— — Id. Siluria, 1st ed., 1854 ; and 2nd ed., p. 184, Foss. 26, 



fig. 2, 1859. 



— — Id. Mem. Geol. Surv., Decade xi, sec. 2, p. 1, pi. ii, 1864. 



S. sescuncialis, ovata, axe angusto ; spicula capitis brevissima. Cajmt semiovatum, 

 ohtusum, glabella ad basim angustd, oculis retrorsis, ad basim capitis pane retractis. 

 Cauda semiovata, obtusa, axe subannulato. 



The general form is depressed and elliptical ; the length about one inch and a half, 

 the breadth one inch. The contour of the head, which is exactly as long as the caudal 

 shield, and more than one third the whole length, is nearly a true semi-oval, evenly convex 

 except on the median line behind (which is abruptly raised), and slopes on all sides 

 to a concave border. The glabella, scarcely defined at all in front, though faintly indicated 

 (more strongly so in young specimens), is of a pyriform shape. Posteriorly it is much 

 contracted, and again suddenly expanded upon the neck-border. Its greatest width 

 behind is not above one fifth that of the head. The eyes are small, convex, much 

 curved, placed at less than their own length from the hinder margin, opposite the 

 contracted part of the glabella, and rather further apart than the Avidth of the thoracic 

 axis. The facial suture runs nearly out nearly at right angles beneath the eye, and 

 in front of it describes a large arc diverging from the eyes at an angle of 70°, and cutting 

 the anterior border far outwards, in a line overhanging the fulcral points. The facial 

 suture is strictly marginal in front, and the hypostome (fig. 4) appears to be quite con- 

 tinuous, without a rostral shield as in lllanus, or a vertical suture as in some Asaphi. 



Two good specimens in Dr. Wyville Thomson's cabinet show the labrum, but its 

 margin is broken off". It is wide at its attachment, considerably convex in the middle, 

 more so than in Asaphus, and is marked with concentric lines on the sides. There is 

 not enough to show that there was no marginal groove, or whether the tip was rounded 

 and entire, as in Illcstms, which is most probable. 



Thorax of nine rings, not so long as the head, and with its axis only two thirds 

 as wide as the pleurse, convex. Pleurae flat as far as the fulcrum, which is about 

 the width of the axis remote from it. Thence the pleurae are bent down and a little 

 back, and facetted for rolling up. There is no groove whatever to the pleurae, which 

 in this respect resemble those of lllanus. 



Tail semi-oval, blunt, not convex ; the conical axis about half the width of the 

 sides, and reaching fully two thirds the length of the tail. The axis has about eight 



