CHEIRURUS KEISLEYENSIS. 45 



number, and originate nnderneatli the head-shield behind the hypostome, diverging 

 radiall^y forwards to the margin, beyond which they project, forming the fringe 

 extending from the axial furrows back to the genal angles. Barrande^ in his second 

 series of figures of Dindymene fred.-augusti, Corda, showed a similar marginal fringe 

 of 12 — 14 minute spines round the sides of the head-shield, but no mention was 

 made of their presence in his description of this species. Lindstrom^ expressed the 

 opinion that D. fred.-augustl was so distinct from the type of the genus, D. haidingeri, 

 that it ought to be removed into a separate new genus. If this is the case, our 

 Girvan species would have to accompany it. The chief distinctions are the follow- 

 ing according to Barrande ^ : (1) In D. liaidingeri the glabella and cheeks are covered 

 with closely set pits, while in D. fred.-avgnsti there is only a scanty granulation; 

 (2) in D. liaidingeri there are no tubercles on the ridge of the pleura, while they 

 are present in the other species ; (8) there is not so marked a thickening of the 

 pleural ridge before the origin of the free spinose point in D. liaidingeri as in the 

 other species. These differences seem trivial and of no generic value, and both 

 species occur on the same stratigraphical horizon in Bohemia. 



Bindijmene cordai has recently been discovered by Mrs. Gray in the Whitehouse 

 Group at Shalloch Mill, which is a new locality for it. 



It is clear that these so-called spines above described in the Thraive Glen 

 specimens are not homologous with the true marginal fringing spines of Acidasjyis, 

 which at first sight they resemble, for they are not attached to the edge of the 

 head-shield, but arise from near the mouth on the lower surface and run forwards 

 in the plane of the head-shield to extend beyond the margin. This feature is only 

 completely visible in one specimen, in which the glabella and one cheek have been 

 broken away, but it is of great importance, and suggests that the " spines " are 

 really of the nature of cephalic appendages, and probably should be regarded as 

 the biramous limbs of a crowded and condensed series of circumoral paired 

 appendages. 



Family Cheirurid.e. 



Genus CHEIRURUS, Beyrich. 

 Cheirurus keisleyensis, Reed. Plate VIII, fig. 1. 



1906. Cheirurus bhmicronatus {j)ars), Eeed, op. cit., pt. iii, p. 138. 



In the light of further and better preserved specimens from the Drummuck 

 Group it appears necessary to remove the supposed examples of Ch. himucronatus 



1 Barrande, ' Syst. Silur. Bohcme,' vol. i, Suppl., p. 117, pi. ii, figs. 11, 12. 



" LindstriJui, " Kesearchcs on the Visual Organs of Trilobites " (' Kongl. Svensk. Vet. Akad. 

 Handl.,' vol. xxxiv, no. 8, 1901), p. 11. 

 ^ Barrande, op. cit., vol. i, p. 820. 



