502 



MISS Gr. L. ELLES ON THE GEAPTOLITE [Aug. 1 898, 



nearer the distal end. They are inclined at an angle of 35° to 40°. 

 The thecal apertures are very slightly concave, and they form with 

 the general direction of the stipe an angle of 120°. One specimen 

 shows the pustules at the thecal bases. 



The English form, then, is characterized by (1) the variable 

 curvature of the stipes ; (2) the rapid increase in width from the 

 initial point ; and (3) the character of the thecse and their number 

 in a given unit of length, 



Occurrenc e. — On one slab it occurs associated with Tetragraptus 

 quadribi^acliiatus (Hall); it probably belongs to the Middle Skiddaw 

 Slates. 



Localities.— Eandal Crag, Skiddaw; east of Longside ; Barf; 

 Bassenthwaite Sand-beds ; Knott Head,Whinlatter Pass; Brundelhow 

 Lead Mine ; Gatesgill ; below E,aven Crag, west of Skiddaw. 



{i) DlDTMOGEAPTUS JSTlCHOLSONI, Lapw. 



1868. I>idymograjpsus serratulus, Nich. {non Hall), Quart. Jouvn. Geol. Soc. 

 vol. xxiv, p. 136. 



1870. Didymograpsxis serraUdus, Nich. Ami, Mag. Nat. Hist. ser. 4, vol. v, p. 343 

 & pi. vii, figs. 3-3 d. 



1875. Didymograjptus Nicliolsoni, Lapw. & Hopk. Quart. Journ. Geol. Soc. vol. xxxi, 

 p. 644 & pi. xxxiii, figs. 5 a-d. 



The Skiddaw Slate specimens now referred to D. Nicliolsoni 

 were provisionally re- 

 ferred by Nicholson Fig. 21. — Impression of Didymograptus 

 to D. serratulus. As Nicholsoni from Barf near Keswick ; 



Lapworth has shown, coll. Woodivardian Museum. 



Hall's species differs 

 from this in the number 

 of thecae in a given 

 unit of length and in 

 their shape. According 

 to Hall's figure, there 

 must be only sixteen 

 thecse in the space 

 of 1 inch (six in 

 10 mm.) in D. serra- 

 tulus, whereas in D. 

 Nicholsoni there are 

 twenty - six (rather 

 more than ten in 10 

 mm.). 



The original speci- 

 mens obtained from 

 the Skiddaw Slates 

 are said to have been 

 imperfect, but some 

 specimens of the form 

 collected from Barf 

 and now in the Wood- 

 wardian Museum are 

 very well preserved. 



[X 1.^] 



