358 J. Sophinson — Structure of GraptoUtes. 



axis, and in the branches, a slight depression. In general form its 

 hydrothec^ mostly resemble those of D. sextans. 



D. Nicholsoni is nearly allied to D. ramosus, but may be readily 

 distinguished from it by the shortness of its stem, the greater diver- 

 gence of its branches, and the curved form of its axil, I have 

 named it after Dr. H. A. Nicholson, vp-ho, by his researches in the 

 Skiddaw Slates and Coniston Flags, has added so much to our 

 knovpledge of the British Graptolites. 



Log. Llandeilo : — Dobb's Linn, Moffat, Dumfriesshire. (In black 

 carbonaceous shale.) 



4. Dicranograptus Clingani, Carr. (1868), Geological Magazine, 

 Vol. v., p. 132, pi. v., fig. 6.— PL XVI., Fig. 4. 



Polypary with a short, nearly parallel, diprionidian stem, dividing 

 acutely into two very short monoprionidian branches, which diverge 

 from each other at an angle of about 40 degrees. Hydrothecse from 

 20 to 25 to the inch, free for nearly two-thirds the width of the 

 polypary ; angular in outline, or but slightly curved ; and closely 

 approximating each other towards their outer margin. Apertures at 

 right angles, or nearly so, to the axis. 



The stem, with its slender radicle and lateral spines, seldom 

 reaches a quarter of an inch in length, and the branches, which are 

 about l-30th of an inch broad, do not exceed half an inch in length. 

 The hydrothecse are somewhat similar in form to those of D. ramosus, 

 but much more frequently appear as "scalariform impressions." 

 They seem almost entirely free, and are much elongated, and there- 

 fore very close together, much closer than in D. ramosus, from which 

 this species also differs in its short stem, and short and slender 

 branches, which never exceed the dimensions I have given. In the 

 specimen illustrated, one branch is seen but very slightly distorted, 

 while the other is so much altered by compression that the apertures 

 of the hydrothecse cannot be seen, the proximal end of each theca 

 being shoved over the distal end of the one which precedes it. 



Log. Llandeilo : — Dobb's Linn, Moffat, Dumfriesshire. 



5. Dicranograptus ramosus, Hall (1865), Grapt, Queb. Group, p. 57. 

 GraptoUthus ramosus, Hall (1847), Pal. New York, vol. i., p. 270, 

 pi. Ixxiii., fig. 3. Diplograpsus ramosus, McCoy (1851), Pal. Foss., 

 p. 8. — PI. XVI., Fig. 5. Cladograpsus inequalis, Emmons (1855), 

 Amer. Geol., vol i. p. 107, pi. i, fig, 12. C. dissimilaris, ib., fig. 15. — 

 PL XVI,, Fig. 5. 



Polypary with a diprionidian stem of considerable length, wider 

 towards the centre than at either end, and dividing acutely into two 

 monoprionidian branches, which diverge from each other at an 

 angle of from 20 to nearly 50 degrees. Hydrothecse about 25 to 

 the inch ; free for half the width of the polypary ; angular or sub- 

 angular in outline ; and separated from each other towards their 

 outer margin by a considerable interval. Apertures forming an angle 

 of about 45 degrees with the axis. 



The stem is, as usual, furnished with a slender radicle and spines. 

 It varies from half an inch to one inch in length, and sometimes 

 attains a breadth of l-8th of an inch at its widest part. The branches 



