350 Dr. H. A. Nicholson on the British 



This well-marked species has long been known to all stu- 

 dents of Silurian geology, but has never been fully described. 

 It is characteristically Upper-Llandeilo, and I am not aware 

 that it occurs in any other formation. One of the most re- 

 markable points about this form is the extraordinary dispro- 

 portion in size between different individuals. Numerous inter- 

 mediate examples, however, occur, connecting the smallest 

 and largest individuals ; so that there can be no doubt as to 

 their specific identity. 



Loc. Upper Llandeilo rocks of various parts of Wales, 

 Abereiddy Bay in Pembrokeshire being one of the most noted 

 localities. Llandeilo rocks of County Meath, in Ireland 

 (Baily). 



Didymograpsus divaricatuSj Hall, sp. PI. VII. figs. 4 & 4 a. 



Graptolithus divaricatus, Hall (Pal. New York, vol. iii. Suppl. p. 513). 

 Dicranograpsus divaricatus^ Hall (Grapt. Quebec Group, p. 57). 

 Didyvm/rapstis elexjans^ Carruthers (in part), Geol. Mag. vol. v. pi. 5. fig. 8a. 

 Didymograpsus Moffatensis, Carruthers, Ann. & Mag. Nat. Hist. Jan. 1859. 



Frond consisting of two long and narrow stipes springing 

 from a mucronate base, attaining each a length of from two to 

 three inches or more, and including between them an " angle 

 of divergence" of from 90° to 130°. The base (fig. 8 ^) is 

 convex and rounded, and is formed by a long triangular me- 

 dian radicle, flanked by two shorter lateral spines, the whole 

 three occupying a non-celluliferous space of over one line in 

 breadth. The radicle is in its normal position on the inferior 

 aspect of the frond, and the celhdes are on the same side of the 

 frond as the radicle. In this species, therefore, as in D. sex- 

 tans^ the true angle of divergence is bounded by the non- 

 celluliferous margins of the stipes. The "radicular angle," or 

 that on the same side of the frond as the radicle, is in this case 

 contained between the celluliferous margins of the stipes, and 

 varies from 270° to 230°. Each stipe is about one-fortieth of 

 an inch in breadth at its commencement, and gradually widens 

 out till a width of half a line may be attained. The cellules 

 are from twenty to twenty-six in the space of an inch, their 

 outer margins curved, convex, and nearly parallel to the axis, 

 the denticles obtuse and rounded, and the cell-apertures form- 

 ing oblique indentations or pouches whicli extend about half- 

 way across the stipe, and are rounded-off internally. Accord- 

 ing to Hall, " the surface is marked by a row of small nodes 

 placed obliquely to the direction of the axis, and situated just 

 below and a little on one side of the bottom of the serrature." 



This beautiful species (originally described by Hall from 

 the Hudson-River group of America) is distinguished from all 



