112 Proceedings of the Royal Ehysical Society. 



Arenig rocks of St David's. They may very properly be 

 dedicated to those scientific investigators who have made 

 these respective formations the subject of special study. 



Sp. 1. Nematolites Grayi, sp. nov. ; Plate II., figs. 4, 5. — 

 Branches of great tenuity, about one-hundredth of an inch 

 in average diameter; slightly and irregularly undulating; 

 bifurcating or giving origin to similar branches at distances 

 varying from half an inch to two inches apart, and anasto- 

 mising and forming an irregular network of large meshes, 

 which are occasionally invaded by a secondary network of 

 smaller and more closely arranged branchlets. 



The characteristic peculiarities of this form are the extreme 

 tenuity of its main branches, and their wrinkled or irregu- 

 larly undulating character. 



Locality — U^pper Ardmillan or Nematolites beds (Upper 

 Caradoc) of Girvan, Ayrshire ; at Shalloch Mill, Penwhapple 

 Glen, Quarrel Hill, etc., in great abundance. 



This species is dedicated to Mr Eobert Gray, F.E.S.E., to 

 whom all palaeontologists are under a heavy debt of obligation 

 for his zeal in the discovery and publication of the Girvan fossils. 



Sp. 2. Nematolites Nicliolsoni, sp. nov. ; Plate II., fig. 6. — 

 Branches rigid, about one-fiftieth of an inch in average 

 diameter; slightly undulating; bifurcating or throwing off 

 similar branches from their convex margin, at right angles to 

 its general direction, sometimes at intervals of half an inch, 

 sometimes in dense groups. 



This form is distinguished from the foregoing in several of 

 its characters. Its branches are much stouter, and the 

 manner of their subdivision is different. The slender branches 

 of Nematolites Grayi can often be followed for long distances 

 without undergoing any marked deflection from the same 

 general direction, and the chief meshes are rhomboidal or 

 triangular in form. In the present species, on the other 

 hand, the new branches are apparently formed by the bifur- 

 cation of the older ones, which consequently repeatedly alter 

 their line of direction, and the resultant meshes are sub- 

 rectangular in form. 



