48 BRITISH GEAPTOLITES. 



Horizon and Localities. — Upper Arenig (Llanvirn of Hicks). 



St. David^s District: Llanvirn Quarry ; Lalce District : Outerside ; Glenderamakin 

 River; Thornsliip Beck ; Barf; Gratesgill ; and Mosedale Beck, near Troutlieck. 



Associates. — It occurs in fair abundance in the Llanvirn beds of South AVales 

 associated with D. hifidvs, D. affinis, and Diplogra/ptus dentatus ; and in the Middle 

 and Upper Skiddaw Slates of the Lake district. 



The type specimens (PI. IV, figs. 5 a, J>) are in the AVoodwardian Museum. 

 Other specimens are in the British Museum (Xat. Hist.), and the private collections 

 of Lap worth and Mr. J. E. Marr, Cambridge. 



Didymograptus artus, sp. nov. Plate IV, figs. 6 a — d. 



Stipes from 12 to 2-j mm. in length, very narrow at their origin, but 



expanding throughout to a maximum of I'd mm. ; diverging at a 



primary angle of about 90°, or with rounded base, and then curving- 



gradually round so that the stipes are approximately parallel. Sicula 



small. Thec^e very closely set, eighteen to nineteen in 10 mm., inclined at 



50°, two to three times as long as mde, overlapijing one half to two thirds 



their length. Apertural margin normal, concave, mucronate. 



Description. — The stijjes are very slender at their origin, not exceeding 



•4 mm. in width, l)ut expand to a maximum of l"o mm. in the first 4 mm. 



As a general rule the stipes arc parallel, and the distance betAveen the 



dorsal walls is from 5 to G mm. ; in some forms, however, an angle of about 



5° is included between the stipes, and these usually show a tendency to come 



together at their distal extremities. 



The sicula is 1'3 mm. long, and is slender and 



Fig. oO. — Didy'ino(jr<tpius artiis, _ ... 



sp. nov. tapering. The first tlieca (tli. 1^) originates shghtly 



above the base of the sicula, and groAvs away from 

 it, leaving it free for some little distance near its 

 aperture. The crossing canal is nearly horizontal, 

 th. 1^ being appressed to the sicula, and then 

 extending beyond it. The origin of the stipes is 

 Proximal end, obverse view. (Cross- symmetrical. Each theca is 1 mm. in length. 



ing canal seen, owing to com- "^ _ ... . , 



pression.) Enlargement of part Affinities. — The SpCCicS is allied to D. Lijldns, but 



' °' ' is smaller, and diifers also in the parallel growth of 



the stipes, the greater uniformity in Avidth, and in the more closely set tliecte. 



Horizon and Localities. — Upper Arenig (Llanvirn of Hicks). L^pper SkiddaAA' 

 Slates. 



St. David's District : Porth-hayog, Ramsey Island. Lalcc District: Thornship 

 Beck; Ellergill. 



