56 Dr. H. A. Nicholson on the Nature and 



The " solid axis " is one of the primitive elements in the 

 formation of every Graptolite. In the simple monoprionidian 

 species it seems to be a solid cylindrical rod (PI, III. fig. 2); 

 but in the bisen-ate forms it is certainly a corneous plate, di- 

 viding the frond into two vertical compartments (PI. III. fig. 3), 

 apparently composed of two laminas, with a median cylindrical 

 rod and perhaps including a central canal. The axis may be 

 prolonged beyond one or both extremities of the celluliferous 

 stipe ; and for convenience I shall term these respectively the 

 basal or '^ proximal " and the terminal or '' distal " extensions 

 "of the axis. These prolongations (PI. III. fig. 4) are little 

 understood ; but they require especial attention, as throwing 

 great light upon the true natm-e of the Graptolitida?. The proxi- 

 mal extension of the axis is present in probably all the time 

 Graptolites, and constitutes the " radicle " or " initial point " 

 of Hall, The radicle varies greatly in length, and it may 

 consist of the solid axis alone, as in Dij>lograj)sus teretiusculus 

 &c. ; or it may be bordered by the common canal prolonged 

 upwards into the first cellule or cellules, as in G. Sagittarius^ 

 Linn., Di])lograpsus cometa^ Gein., and D. acuminatus^ Nich. 



Again, in most of the branching and complex Graptolites 

 the solid axes of the various simple stijDCS composing the frond, 

 together probably with the common canal, are prolonged proxi- 

 mally and are united into a connecting process, which is always 

 destitute of cellules and is termed the ^' funicle " by Hall, as is 

 seen in the genera DicJiograj>sus and Tetragrapsus (PI. Ill, 

 figs. 5, 6, 15, 16, and 20). 



Lastly, the subdivisions of the funicle may be embraced by 

 a central corneous disk or cup, which is apparently composed 

 of two laminaj, and probably enclosed a central cavity. This 

 corneous cup (fig. 6) is best seen in several species of Dicho- 

 grapsus and Tetragrapsxis ; but it also exists in some specimens 

 of Diplograpsus hicorms^ Hall (figs. 8, 9, 10), and in a new 

 species of Diplograpsus from the Upper Llandeilo rocks of 

 Dumfriesshire, which I have named D. jjliysophora (fig. 7). 

 The distal extension of the solid axis is only seen in the Di- 

 plogra/psi^ and possibly in Betiograpsus^ Hall, and RetioUtes^ 

 Ban-., seeming to be merely accidental when seen in the adult 

 monoprionidian forms, as it rarely is. This distal extension 

 of the axis usually consists of the solid axis alone, as in all 

 the common Diinlograpsi ; but it may consist of a bladder-like 

 body, more or less elliptical in form, with a distinct filiform 

 margin and of uncertain function. This vesicular dilatation 

 seems always to be a direct expansion of the axis, which would 

 thus appear to be tubular. It is seen to a very moderate ex- 

 tent in some specimens of D. jpristis, His., and D. pahneus, 



