DIDYMOGRAPTUS. 



25 



grows out at once, so that the first stipe appears to originate at that point ; but 

 the second theca (tli. 1") grows out close to the aperture. 



The thecfB are in contact only ; the width at their apertures is twice that at 

 their origin, and they are three times as long as wide. The average length of 

 each theca is 1"G mm. 



Affinities. — 1). yracili^ is nearly related to D. 

 ajfinis, but is distinguished by the horizontality of 

 the stipes and the more remote thecge. 



Horizon and Localities. — Arenig, Middle Skiddaw 

 Slates. 



Ldlrc District : Barf, near Keswick. 

 There are very few British specimens known ; 

 the best is in Postlethwaite's collection at Keswick, 

 and all those recorded up to the present time have 

 ])een found in the Lake district. 



Figs. 14 o a,nd b. — Dkhjmogvaptus 

 gracilis, Toviiquist. 



.'iV^ 



a. Proximal end, obverse view ; en- 



largement of part of PI. II, fig. 2. 



b. More distal thecse of same speci- 



men. 



Didymograptus acutidens, Lapw. MS. Plate II, figs. 8 a — (/. 



1875. Bidymograptus affinis, Hojjkinson, Quart. Jouru. Greol. Soc, vol. xxxi, pi. xxxiii, figs. 6 h, c. 



Stipes 5 — 7 cm. or more in length, narrow at origin but widening steadily 



to a maximum of 1"5 mm., diverging from a conspicuous sicula at angles of 



130° — 150°. Thecse eleven to thirteen in 10 mm., four times as long as wide, 



overlap and inclination varying in different parts of the stipe. Apertural 



margin slightly oblique, concave, and with distinct denticles. 



Description. — The stipes are exceedingly narrow at their origin, being often not 



more than "4 mm. in width ; they widen steadily up to a maximum of 1*5 mm., 



but only measure '6 mm. opposite the apertures of theca 10 or theca 11. The 



stipes are commonly straight, but may exhibit slight curvature in the region of the 



sicula. 



The sicula is small and narrow thoug-h con- 

 spicuous ; it is placed somewhat obliquely in the 

 same manner as D. Nicholsoni ; theca 1' appears 

 to originate about halfway up the sicula on the left 

 side (obverse aspect) ; it grows out at once and the 

 crossing canal is very short, so that theca 1^ arises 

 almost at the same level as theca 1^ ; it also curves 

 away at once from the sicula, which thus has the 

 whole of its apertural region free. The attachment of the stipes to the sicula is 

 slender. 



4 



Fig. lo u. 



-Dichjmograptiis acutidens, 

 Lap worth MS. 



a. Proxiiiuil end, reverse view. Portli- 

 haj'og, Ramsey Island. Coll. 

 Woodwardiau iluseum. 



