DIDYMOGRAPTUS. 



19 



so oblique as indicated by Hall's figures. The apex of the denticle of each aperture 

 is vertically above the fourth or fifth theca in advance. 



Affiuifies. — The peculiar form of the stipe and the great length of the thecse 

 render this species easily recognisable, and by these same characters it can readily 

 be distinguished from all other forms. 



Horizon and Localities. — Lower Arenig (of Hicks). 



St. DavitVs District : Road Uchaf, Ramsey Island. 



Associates, etc. — It occurs in association with D. extensus and D. sjyarsas in the 

 Arenig rocks of the St. David's district. Specimens collected from that area by 

 Hopkinson are now in the Woodwardian Museum. 





Characters 



of the Group of D. 



extensus. 









B. extensus. 



D. nil id IIS. 



D. nniformis. 



Horizontal 



(but with 



marked 



proximal 



curvature) 



D. paiulus. 

 Horizontal 



D. hirundo. 



D. sparsus. 



D.pennatulus. 



Form of stipe . 



Horizontal 



(or with 



slight 



curvature) 



Horizontal 

 (or with 

 proximal 



curvature) 



Horizontal 



Horizontal 



Horizontal 



Width 



1'6 mm., 



1-6- 21 



1-6 mm., 



21 mm.. 



21 — 4 mm., 



3 mm., 



76 mm.. 





increase 

 gradual 



mm., 

 increase 

 gradual 



uniform 



increase 

 gradual 



increase 

 rapid 



increase 

 gradual 



increase 

 very rapid 



Length of sicula 



13 mm. 



13 mm 



18 mm. 



2'1 mm. 



3'2 mm. 



3-2 mm. 



— 



Origin of stipes . 



Apical 



Apical 



Apical 



Apical 



Apical 



Apical 



Apical 



No. of thecse in 

 10 mm. 



9—10 



12-13 



11 



9—10 



9—10 



7 



9—10 



Inclination 



40° 



40° 



30° 



60° 



60^ 



45° 



35o_60^ 



Overlap 



1 2 



2 3 



1 2 

 2- 31 



1 2 



2 3^ 



i 



1 



\ 



f 



Horizontal Seeies {continued). 

 Group II. — Type I), superstes. 

 Didymograpti in which the stipes are curved proximally but are distally hori- 

 zontal ; they originate a short distance above the aperture of the sicula. The thecae 

 are inchned at moderate angles, and the amount of overlap is never great. 



Didymograptus superstes, Lap worth. Plate I, tigs. 9 a,h. 



18 76. Didymograptun sujjerstes, Lapworth, Cat. West. Scottish Fossils, pi. iii, figs. 74 a, b. 

 1877. Didymograpttis superstes, Lapworth, " Oraps. of Co. Dowti," Proc. Belfast Nat. Field Club, 

 p. 142, pi. vii, figs. 15 a, b. 

 Stipes several cm. long and gracefully curved, narrow at origin, but increasing 

 rapidly in width to a maximum of 3 mm., diverging from a small sicula at 



