356 J. HopUnson — Structure of Graptolites. 



then appears in Ireland. In the Hudson Eiver Group of Canada 

 and New York, two British species, D. ramosus and D. sextans, are 

 found, associated with another species, D. furcatus (Hall), not known 

 to occur in Britain. D. ramosus also extends to the Lower Silurians 

 of Australia, thus having a very great geographical range. 



The genus is represented in Britain by the following species : — 



1. Dicranograptus sextans, Hall (1865), Grapt. Queb, Group, p. 57. 

 Graptolithus sextans, Hall (1847), Pal. New York, vol. i., p. 273, 

 pi. Ixxiv., fig. 3. Diplograpsus (^ Didymograpsus) sextans, McCoy 

 (1851), Pal. Foss., p. 9.— PL XVI., Fig. 1. 



Polypary with a.n extremely short diprionidian stem, dividing into 

 two short and slender monoprionidian branches, which diverge from 

 each other at an angle of about 60 degrees. Hydrothecae from 30 to 

 35 to the inch; free for fully half the width of the polypary; 

 curvilinear in outline, but with an outer margin nearly straight 

 towards the distal end. Apertures situated on the convex distal 

 margin of each hydrotheca, and forming an angle of about 45 

 degrees with the axis. 



The first two hydrotheca only are united, forming an undivided 

 portion which can scarcely be called a stem, for it is not more than 

 l-30th of an inch long, exclusive of the minute radicle and slender 

 lateral spines. The branches are from one-third to nearly half an 

 inch long, and about l-30th broad, and are remarkable for the con- 

 stancy of their angle of divergence, which is the same in both 

 American and British specimens. The hydrothecee are decidedly 

 curved in their free portion, and in British specimens have no 

 mucronate point, as described and figured by Hall. They are nearly 

 parallel with the axis in their outer margin, are slightly rounded off 

 to the aperture, and prolonged untU forming an angle of from about 

 30 to 40 degrees with the axis. 



The position of this species is somewhat doubtful, but it seems 

 nearer to Dicranograptus than to the allied species of Bidymograpsus, 

 with which it is usually associated. The stem is certainly exceedingly 

 short — almost rudimentary ; but it bears a fair proportion to the 

 entire polypary, being nearly 1-lOth the length of the branches; and 

 there is no indication of a spine opposite the true radicle, as in the 

 most nearly allied species of Bidymograpsus, which, moreover, have 

 very long branches. 



Loc. Arenig : — Braithwaite Brow, Skiddaw, Cumberland (Salter) ? 

 Llandeilo : — Builth, Wales ; Glenkiln Burn, Dumfriesshire ; Loch 

 Eyan, Wigtownshire ; Kilmacreagh, etc., Clare, Ireland (Baily). 

 Caradoc : — Canada and New York (Hudson Eiver Group). 



2. Bicranograptus formosus, sp. nov. — PL XYL, Fig. 2. 

 Polypary with a very short diprionidian stem, dividing into two 



slender monoprionidian branches, which diverge from each other at 

 an angle of from about 30 to 45 degrees. Hydrotheca from 30 to 35 to 

 the inch ; free for a third, or sometimes nearly half the width of the 

 polypary ; curvilinear in outline, and more or less incurved towards 

 the distal end. Apertures situated on the convex distal margin of 

 each hydrotheca, and forming but a slight angle with the axis. 



