66 TRILOBITES OF GIRVAN. 



being only distant from tlie axis about half its width. The lateral angles are only 

 slightly truncated, being cut off obliquely at only about 30° to the front edge. 

 There is no groove behind the fulcrum. In /. murchisoni, as figured by Salter, the 

 pygidium is more convex, and there is a groove behind the fulcrum. 



There is also a free cheek from the same locality which may be referred to the 

 same species. It is of trapezoid form with the eye large and far forward; the 

 anterior and posterior 1 tranches of the facial suture meet at about 110°, the 

 anterior branch curving forwards and outAvards nearly at right angles to the 

 lateral margin ; the genal angle is about 60° but rounded, and the surface near it 

 is concentrically wrinkled. 



There is likewise an epistome from Craighead which may belong to the same 

 species. It is of transverse shape, about twice as wide as long, with the anterior 

 edge flattened but the posterior a\gc arched regularly, and the surface crossed by 

 about fifteen transverse stria 1 . It is not angulated behind, nor does it bulge out 

 suddenly in the middle as in J. davisi and some other species; it is longer and 

 tapers more rapidly to the lateral angles than /. thomsoni, and the transverse stria? 

 are finer and more numerous. 



The pygidium from Penkill attributed by Nicholson and Etheridge to I. m urcli ison i 

 (M., fasc. ii, 1879, p. 101, pi. xii, fig. 1) is probably referable to I. thomsoni. 



Collection . — Mrs. Gray. 



Horizon and Locality. — Stinchar Limestone Group (Llandeilo) : Craighead. 



10. Illgenus nexilis, Salter, 1807. Plate IX, fig. 13. 



1867. Illsenus nexilis, Salter, Mon. Brit. Trilob., p. 190, pi. xxx, figs. 4, 5 (described as a sub-species of 



I. bowmani). 

 1877. Illxnus nexilis, Woodward, Cat. Brit, Foss. Crust., p. 158. 

 1879. Ilhenus nexilis, Nicholson and Etheridge, Mon. Silur. Foss. Grirvan, fasc. ii, p. 158, pi. xi, fig. 11 ? 



(non figs. 12, 13). 

 1882. Illienus nexilis, Holm, Svensk. Art. Illsenus (Bib. K. vet. Akad. Handl., vol. vii, No. 3), p. 48. 

 1899. Ilhenus nexilis, Mem. Geol. Surv., Silur. Rocks Brit,, vol. i, Scotland, pp. 536, 673, 689. 



Remarks. — This species was only recorded from Mulloch by Salter (pp. cit), 

 who gave it its name and considered it to rank merely as a sub-species intermediate 

 between I. bowmani and I. thomsoni. The epistomes attributed to it are certainly 

 distinct from those of either species, but the head, thorax and pygidium are scarcely 

 distinguishable from /. thomsoni, particularly when imperfect or in a poor state of 

 preservation. 



Collections. — Mrs. Gray; Museum of Practical Geology. 



Horizons and Localities. —Mulloch Hill Group (L.Llandovery): Rough Neuk ; 



