158 TBILOBITES OF GIRVAN. 



less than one third the width of thorax at about the sixth ring, behind which it 

 gently tapers to pygidium ; gently convex ; each ring with low but distinct nodular 

 swelling on axial furrows. Axial furrows strong and deep. Pleura? curved gently 

 backwards and downwards beyond the weak fulcrum which is situated at about half 

 their length. Strong oblique furrow along each pleura. 



Pygidium transversely parabolic, about one and a half times as broad as long. 

 Axis conical, about one third the width of pygidium and reaching nearly its 

 whole length, tapering gradually to a rounded extremity ; composed of twelve 

 distinct rings followed by four narrow faint ones. Lateral lobes strongly bent 

 down, especially near margin ; bearing one half pleura on front edge followed first 

 by eleven pairs of regular pleura? (each with very faint, central, impressed line), 

 and then by four pairs of very weak narrow pleurae and by one median unpaired 

 one which runs nearly straight back behind axis. Posterior margin of pygidium 

 between last four pairs of pleura? strongly bent up and appearing from behind 

 almost angulated in centre. Surface of thorax and pygidium finely granulated. 

 Dimensions. — 



Length of head-shield . . . .16-0 mm. 



Width „...-. 37-0 „ 



,, of glabella at base . . . 9"0 ,, 



,, „ in front . . . 23'0 ,, 



Length ,,.... 13*5 „ 



„ of frontal lobe of glabella . . . 8*5 „ 



,, of eye .... 5*0 ,, 



„ of thorax .... 25*0 ,, 



Width „ . . . . 35-0 „ 



,, of axis of thorax . . . 110 ,, 



Length of pygidium .... 18"0 „ 



Width „ . . . . 25-0 „ 



Length of axis of pygidium . . 15*0 ,, 



Width „ ... 9-0 „ 



Remarks and Affinities. — The above description is drawn up from a remarkably 

 perfect specimen of a complete individual which has been most kindly lent me for 

 examination by Mr. James Bisset, F.G.S., of Edinburgh. The specimen came 

 from the Drummuck Beds of Lady Burn, and (with the exception of one head- 

 shield recently discovered by Mrs. Gray) is the sole representative of the subgenus 

 Chasmops which I have seen from Girvan. It measures nearly 60 mm. in total 

 Length, and appeal's to belong to a new species. 



This specimen cannot be referred to P. macroura, Sjogr., as interpreted by 

 Salter, 1 because of (1) its differently shaped glabella with its greater anterior 

 expansion; and (2) its shorter and more transverse pygidium with fewer segments. 

 1 Salter, ' Mon. Brit. Trilob.,' p. 37, pi. iv, figs. 18—23. 



