356 On the Brifi'.th Species of Didymograpsus. 



tioii to DicrunograpsuSj occurs also in J), anccps^ and is not 

 sutlicient to require the removal ot" the sj)eeies Ironi Didipno- 

 qrapsus. Recently, however, Mr. John llopkinson has been 

 kind enough to send me drawings of some specimens which 

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

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

 wliilst the bulk of the frond has all the characters of D. sc.c- 

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

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

 Whether this form is identical with GraiitoUthus farcatus, 

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

 should be looked upon as a transition between I), 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, 



Loc. Abundant, but badly preserved, in the anthracitic 

 shales of Glcnkiln Burn in Dumfriesshire, and Cairn Ryan in 

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



EXPLANATION OF PLATE VII. 



Fig. 1. Didymograpms patulus, Hall, nat. size. From the Skiddaw Slates 

 of Outerside, near Keswick. 



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

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



Slates of Outerside, near Keswick. 



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



Hall. 

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

 Slates of Outerside, near Keswick. 



3 n. 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. 

 fid. D. serratulus (?), from the Skiddaw Slates of Thoraship 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- 

 friesahire specimen. 



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



rocks of Dumfriesshire. Enlarged. 

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

 rocks of Dobbs's Linn, near Molfat. 

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

 men there is no radicle. 

 n 6. Base of another specimen of the same, in which a radicle is pre- 

 «ient ; enlarged. 



