40 TRILOBITES OF GIRVAN. 



from the same horizon which Mrs. Gray has recently obtained, the sixth thoracic 

 segment is seen to possess a long, straight median spine on the axial ring pro- 

 jecting backwards horizontally to about the tenth or eleventh ring, and the pleurae of 

 the sixth segment are likewise furnished with a similar parallel spine of apparently 

 the same length slightly curved inwards, arising at about two-thirds the length of 

 the inner part of the pleura from the posterior ridge on its surface and directed 

 backwards. In one of the examples showing these characters the hypostome is 

 exposed in position and possesses the typical characters of the genus, the body 

 being elongated and slightly pear-shaped, strongly convex and narrowing 

 anteriorly. 



In all respects, except the presence of the thoracic spines, these new specimens 

 (which have the head-shield well preserved) agree with the form previously 

 described. Portlock's species was based on the pygidium, and with it the pygidia 

 of the Girvan specimens have been shown to agree very closely. The thorax was 

 not known to Portlock, but the head-shield described by him as Avijji/x ? haccatns 

 [up. cif., p. 262, pi. iii, fig. 11) was referred first by Salter in 1853 to the same 

 species. There does not seem sufficient reason for separating the Girvan form 

 specifically, but it may provisionally be regarded as a variety under the name 

 trispinosus. In E. seebachi, Schmidt,^ long median spines are known to exist on 

 several successive axial rings of the thorax, and Schmidt remarks that as in some 

 specimens there is no trace of them, they may only be a sexual distinction. But 

 none of the pleurse possess spines similar to those in our Girvan form. 



Genus CYBELE, Loven. 

 Cybele loveni, Linnarsson, var. girvanensis, Reed. Plate VII, figs. 4, 5. 



1906. Cybeh: loveni, var. girvanensis, Eeed, op. cit., pt. iii, p. 126, pi. xvii, figs. 1 — 4. 



In the previous description of this variety there was no account given of the 

 eye, as it was not satisfactorily known. But an unusually well-preserved head- 

 shield from the Starfish Bed, measuring 13*5 mm. in length and 34 mm. in width, 

 shows this organ almost uninjured, and the extraordinary height to which it rises 

 can be measured. The eye, in fact, though not mounted on a stalk, projects as a 

 subcylindrical rod nearly at right angles to the surface of the cheek to a height 

 of 3'25 mm., while its diameter is about 1*75 mm. ; the outer and lateral faces of 

 the whole of this rod-like prominence are rounded and bear the lenses, while the 

 inner side is flattened and formed by the long narrow eye-lobe. 



Another specimen from the same bed deserves notice because it shows the 

 1 Schmidt, ' Eev. Ostbalt. Silur. Trilob.,' pt. i, p. 229, pi. xiv, figs. 16—26 ; pi. xv, figs. 21—23. 



