18 BRITISH GRAPTOLITES. 



growth lias the effect of almost completely concealing the greater part of the sicula 

 in the reverse aspect, leaving it visible only near its aperture. 



The thecEe are always in contact for half their length, and have a uniform 

 inclination of about 45° ; their outer walls are straight, and their apertures concave ; 

 in the fully developed parts of the stipe the apertures appear to be perpendicular 

 to the general direction of the stipe. The thecae average seven in 10 mm. ; they 

 are markedly more remote than those of any other form with which this species is 

 associated. Each individual theca measures 3"2 mm. 



Affinities. — The proximal end of this species is similar to that of D. hirundo, but 

 the remote thecae in the distal parts of the stipe serve to distinguish it. It may 

 easily be separated from the other forms with which it occurs associated, namely, 

 D. 'pennatulus and D. eMeasiis, by its large sicula and remote thecae. 



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



St. David's District : Road Uchaf , Ramsey Island. 



Associates, etc. — The species occurs associated Avith D. pentiatulus and D. extensus 

 in the cleaved slates of the St. David's district, and several specimens originally 

 collected by Hopkinson are now in the Woodwardian Museum, Cambridge, as are 

 also the type specimens (PI. I, figs. Gnr, h). 



Didymograptus pennatulus (Hall). Plate I, fig. 7. 



1865. Graptolithus pennatuhis, Hall, " Grapt. of Quebec Group," p. 82, pi. iii, figs. 1 — 8; pi. v, fig.9 (?). J% 

 1875. Didymograptus 'pennatulus, Hopkinson, Quart. Journ. Geol. Soc, vol. xxxi, p. 643, pi. xxxiii, 

 figs. 3 a — e. 



Stipes several cm. in length, very narrow at origin, but widening abruptly to a 



breadth of 3"2 mm., and then gradually to 6*4 mm. ; diA^erging at 180° from 



the sicula. Tliecse nine to ten in 10 mm., inclined at a large angle, eight times 



as long as wide, free one fourth their length. Apertural margin concave. 



Description. — The stipes widen very abruptly for the first G mm. from a width 



of 1 mm. to 3*2 mm. ; thereafter the increase is very gradual up to a maxinuun of 



7"6 mm. in the widest forms, but rather less in others ; there is a slight diminution 



in breadth at the distal extremities, owing to the hicomplete development of the 



latest formed thecae, producing an apparent curvature of the thecal margin. 



The sicula has not been observed in any British specimens, — in fact, only isolated 

 stipes have as yet been recognised ; but the form of the stipe and the general cha- 

 racters of the thecae leave no doubt as to their identity with Hall's species. 



The proximal thecae are very small, measuring barely 2*1 mm., but those in the 

 mature part of the stipe are very long and narrow, measuring G"3 mm. The walls 

 are concavely curved, so that while the initial inclination may be as low as 35°, near 

 the aperture the theca is inclined at about 60°. The apertures are small and distinctly 

 mucronate ; the apertural angle, after compression, is about 100° — 110°, which is not 



