356 On the British Species of Didymograpsus. 



tion to DicranograjJsuSy occurs also in D. a7icej}s, and is not 

 sufficient to require the removal of the species from Didymo- 

 grapsus. Recently, however, Mr. John Hopkinson has been 

 kind enough to send me drawings of some specimens which 

 appear to belong, beyond a question, to D. sextans, but in 

 which this amalgamation has gone further. In these, namely, 

 whilst the bulk of the frond has all the characters of D. sex- 

 tans, there is an exceedingly short basal portion formed by a 

 coalescence of the first two or three cellules on each side. 

 Whether this form is identical with Graptolithus farcatus, 

 Hall (Pal. New York, vol. i. pi. 74. figs. 4a-/i), or whether it 

 should be looked upon as a transition between D. sextans and 

 Dicranograpsus proper, I am unable to say. D. sextans, in 

 its typical form, as above described, is easily recognizable by 

 the shortness of the stipes, the constancy of the angle of di- 

 vergence, the presence of the radicle and the cellules on the 

 same side of the frond, and the characters of the cellules. 



Log. Abundant, but badly preserved, in the anthracitic 

 shales of Glenkiln Burn in Dumfriesshire, and Cairn Kyan in 

 Ayrshire; also in several localities in Ireland (Baily). 



EXPLANATION OF PLATE VII. 



Fig. 1. Didymograpsus pntulus, Hall, nat. size. From the Skiddaw Slates 

 of Outerside, near Keswick, 



1 a. Portion of JD. patulus, enlarged, to show the cellules, after Hall, 

 Fig. 2. Didymograpsus extensus, Hall, nat, size. From the Skiddaw 



Slates of Outerside, near Kesw-ick, 



2 a. Fragment of D. extensus, enlarged, to show the cellules, after 



Hall. 

 Fig. 3, Didymograpsus serratulus, Hall, nat, size. From the Skiddaw 

 Slates of Outerside, near Keswick. 



3 a. Base of D. serratulus, enlarged, after Hall. 



3 b. Base of D. serratulus, from another specimen, from the Skiddaw 

 Slates of Outerside, Enlarged. 



3 c. D. serratulus (?), from the Skiddaw Slates of Outerside, natural 



size. The angle of divergence is much greater in this than in 

 ordinary specimens. 

 3f?, D. serratulus (?), from the Skiddaw Slates of Thornship Beck, 

 near Shap. The angle of divergence in this specimen is much 

 less than in ordinary specimens. Natural size. 

 Fig. 4. Didymograpsus divaricatus, Hall, slightly restored from a Dum- 

 friesshire specimen. 



4 a. Base of a specimen of D. divaricatus, from the Upper Llandeilo 



rocks of Dumfriesshire. Enlarged. 

 Fig. 5. Didymograpsus onceps, Nich., slightly enlarged. Upper Llandeilo 

 rocks of Dobbs s Linn, near Motfat. 



5 a. Base of another specimen of the same, enlarged. In this speci- 



men there is no radicle. 

 5 b. Base of another specimen of the same, in which a radicle is pre- 

 sent : enlarged. 



