STYGINA. 173 



faint furrows. The sides are gently convex at first, and then broadly concave, with 

 a somewhat sharply defined margin ; it is without any furrows, even the usual upper 

 one is obsolete, or nearly so. The apex is very blunt, more so than the front of the 

 head. The incurved striated portion is broad, and not indented by the point of the axis. 

 Locality. — Caradoc of Desertcreight, Tyrone. It will probably be found in the 

 South of Scotland. 



Stygina Murchisoni^, Murchison. PI. XVIII, fig. 11. 



Ogygia MuRCHisoNi^, MurcMson. Silur. Syst., pi. xxv, fig. 3, 1837. 

 Stygina Murchisoni^, Salter. Rep. Brit. Assoc. Traus. Sect., p. 59 (read 1852), 



, 1853. 



— — Id. Morris's Catal., 2iid ed., Foss. 10, fig. 4, p. 55, p. 



115, 1854. 



— — Id. Siluria, 2ud ed., p. 55, Foss. 1 0, fig. 4 ; pi. iv, fig. 1, 



1859. 



— — Id. Mem. Geol. Surv., Decade xi, sec. 2, p. 3, 1864. 



S. convexa, valde trilobata, capite longe semiovato cum sjnculis j^t'oductis. Cauda 

 longe semiovata, axe prominulo Icsvi, non aunulato. 



In the black schists, which alternate with conglomerate and grit-bands at Mount 

 Pleasant, Carmarthenshire, and wiiich are most probably of Caradoc, not of Llandeilo 

 age, a single specimen of this rather remarkable fossil occurred to Sir Roderick 

 Murchison's hammer, and none other has since been found. The locality is worth 

 searching. Nacula and Orthoacras are found with it. 



The head is greatly longer in proportion than that of the species last described, 

 forming a long semi-oval, produced behind into lengthy spines, and divided strongly 

 into glabella and cheeks at the base. Above, our only specimen is indistinct, and it does 

 not show the position of the eyes. 



Of the thorax-rings but three or four joints are preserved, but these show the 

 trilobation to be very strongly marked, and the axis is quite convex, while the pleurae 

 are considerably bent down from the fulcral point, and backwards too. They seem 

 to be longer and narrower than those of S. latifrons. 



The tail, reversed on the specimen (which was probably curled up when imbedded 

 in the silt) is also of a long-oval contour, and its narrow axis is prominent to the 

 very end ; it shows no signs of transverse ribbing. The sides are faintly convex, then 

 concave along the margin, which reveals, when the upper crust is removed, a moderately 

 broad fascia, closelv striated. 



The species differs at a glance from S. latifrons in the much longer shape and 

 more protracted head-spines. The tail, too, has a longer axis, without a trace of annular 

 furrows, and the trilobation is throughout much more distinct. 



