346 Dr. 11. A. Nicholson on the British 



parallel, being bent towards the middle line so as to diniiuish 

 the primary angle of divergence. 

 The average length of the stipes ^'S'^ ^• 



is not more than a quarter of an 

 inch ; but in rare cases more than 

 half an inch may be attained. 

 The -width of the stipes is very 

 unifoiTn. The cellules are on the 

 opposite side of the frond to the 

 radicle, or occupy the sides of the 

 angle of divergence ; they are 

 about thirty in the space of an Didymofp-ajmis yeminm, His., 

 inch, the denticles angular, and from the Skiddaw Slates : o, an 

 tlie cell-mouths at ri-ht angles to ^\""«"f b' larfre specimen, nat. 

 , - ,• 1 • riM 1 1 Size : 0, another specimen, en- 



tlic axis ot the stipe. 1 he length ij.i.ged and with the cellules 

 of the radicle is from one-twelfth partially restored, 

 to one-tenth of an inch. 



D. geminus is an unmistakable species, being at once reco- 

 gnized by the general shape of the frond (something like that 

 of a tuning-fork), in which it differs from all other forms. 

 Didymograjjsus [Grajjtolitl/us) indentus, Hall (Grapt. Quebec 

 Group, pi. 1. fig. 20), is probably a large example of this spe- 

 cies ; other-\vise the form docs not appear to be represented in 

 the Silurian rocks of America. I), geminus is exti*emely 

 abundant in some beds of the Skiddaw Slates ; but it is very 

 rare to find any .specimen in which the form of the cellules is 

 exliibited. The larger examples of the species approximate 

 to the smaller forms of I), hifidus, Hall, and D. Jfurr/iisoni, 

 Beck ; but the sha])e of the cellules is sufficiently distinctive. 



Loc. Skiddaw Slates : Outerside and Bai-ff, near Keswick ; 

 Bannerdale Fell, near Mungrisedale ; Thornship Beck, near 

 Shap (upper beds). Lower Llandeilo : Cefn Gwynlle ; Shelve, 

 Shropshire. 



Didymograjisus hijidus^ Hall, sp. Fig. 7. 



Graptolithus bifidus, Ilall (Grapt. Quebec Group, p. 73, pi. 1. figs. lG-18, 



pi. 3. figs. 9, 10). 

 Didijmograpsiis bijidus (Nicholson, Quart. Journ. Geol. Soc. vol. xxiv. 



p. 13Gj. 



Frond composed of two stipes diverging from a short blunt 

 radicle at an angle of from 15° to 30° (as much as 60° in a speci- 

 men figured by Hall). The length of the stipes varies from a 

 quarter of an inch to one inch ; and the breadth varies in different 

 parts of the stipe. Towards the base each stipe is very naiTow; 

 but it gradually expands till a Avidth of a line may be attained 

 (from one-eighth to one- quarter of an inch in American ex- 

 amples), and then a gradual diminution of width takes place 



