56 TRILOBITES OF GIRVAN. 



angles are truncated at about 60° to the front edge, and there are 6 — 7 strong strice 

 crossing the surface from side to side. 



Collections. — Mrs. Gray (f . M.) ; Museum of Practical Geology ; Edinburgh 

 Museum ; ? Hunterian Museum. 



Horizon and Localities. — Saugh Hill Group (M. Llandovery) : Woodland Point. 

 Camregan Group (U. Llandovery) : Bargany Pond Burn. Penkill Group (Tarannon) : 

 Penkill ; Penwhapple Glen. 



2. Illaenus balclatchiensis, sp. nov. Plate VIII, figs. 12 — 16. 



Specific Characters. — Pygidium broadly oval, nearly half as wide again as long, 

 very slightly arched from side to side, but steeply inclined near posterior margin. 

 Lateral angles truncated at 45° — 60°, but gently arched outwards, not straight. 

 Fulcrum distant from axis about two thirds its width, with very faint shallow 

 groove behind it. Axis about one third as wide and about half as long as 

 pygidium ; subcylindrical, with slight independent convexity, raised above lateral 

 lobes and projecting on front margin. Axial furrows broad, shallow, straight, 

 subparallel, dying out posteriorly before end of axis (in casts they are seen to be 

 united behind axis, which has a rounded extremity as in I. latus). Low narrow 

 ridge in some specimens runs back on surface of shell from end of axis to posterior 

 margin. In casts a groove is seen behind axis running straight back towards 

 margin. Caudal fascia wide, about one third to one half the length of pygidium ; 

 of few rather widely spaced striae. Surface of shell ornamented with minute 

 punctae, with scattered larger pits. 



Dimensions. — 



Length of pygidium 



Width „ „ across broadest part 

 ,, of axis 



Remarks. — The species I. boivmam, Salter, to which the specimens here described 

 as a new species have been previously assigned, has been frequently used as a kind 

 of refuge for all sorts of forms which owing to their imperfect condition or bad 

 state of preservation were difficult of discrimination. This has had the unfortunate 

 result of obscuring the definiteness of the English species, and rendering the com- 

 parison of it with foreign forms a matter of difficulty. Holm 1 has been led 

 accordingly to stigmatise the British species as badly characterised. It is therefore 

 better to separate off examples with differences of a marked character, though these 



1 Holm, ' Kev. Ostbalt. Silur. Trilob.,' pt. 3 (1886), p. 153. 



I. 



II. 



12-0 mm. 



13-0 mm 



17-5 „ 



19-0 „ 



7-0 „ 



8-0 „ 



