24 Dr. H. A. Nicholson on Helicograpsus, 



they form two distinct sets, which diverge in opposite direc- 

 tions. The extremities of the funicle, where the branches 

 cease to be given off, become themselves also celluliferous on 

 one side ; and in the centre of the funicle a small radicle may 

 occasionally be detected. The celluliferous branches do not 

 subdivide or give origin to secondary branches, as far as has 

 been observed. It is probable that the perfect polypary was 

 composed of two fronds, such as above described, placed trans- 

 versely across each other in a cruciform manner ; and though 

 none of our English examples would support this view, such a 

 specimen has, according to Hall, been discovered in America 

 (Grapt. of the Quebec Group, p. 14, note). 



The above characters combine to form a Graptolite so essen- 

 tially distinct from all others, that there can be no hesitation 

 in forming a new genus for its reception. By Hall it was 

 placed in his genus Graptolithusj in accordance with the be- 

 lief which led him to place Dichograpsus^ Tetragrapsus^ and 

 Didymograpsus in the same genus — the belief, namely, that 

 there existed in nature no such simple forms of Graptoutes as 

 G, Sagittarius^ Linn., O. Sedgwichiij Portl., &c. The refer- 

 ence to Rastrites was founded upon imperfect fragments, and 

 has long ago been given up by its author. There remains, 

 then, only the reference to Cladograpsus by Mr. Carruthers ; 

 and a short consideration will show that this is certainly in- 

 applicable. In the genus Cladograpsus (originally founded 

 by Geinitz to include certain Didymograpsi) Mr. Carruthers 

 placed, some years ago, a peculiar branching Graptolite, which 

 he described under the name of C. linearis (Ann. & Mag. Nat. 

 Hist. ser. 3. vol. iii. No. 13). This he subsequently abandoned, 

 placing the form in question under the genus Dendrograpsus, 

 Hall (Geol. Mag. vol. iv. No. 2. p. 70). It was then described by 

 myself as the type of a new genus, under the name of Pleuro- 

 grapsus linearis {ibid. vol. iv. p. 256) ; and I at that time pointed 

 out that the essential point in the definition of the genus, where- 

 by it was distinguished from all other branching Graptolites 

 known to me, was the entire absence of a " funicle," or non- 

 celluliferous basis, the frond consisting of a main celluliferous 

 rachis giving off celluliferous branches, which in turn gave 

 origin to secondary branches. Finally Mr. Carruthers re- 

 turned again to the genus CladograpsuSj redefining it as fol- 

 lows : — " Polypary compound, growing bilaterally from the 

 primary point, irregularly and repeatedly hranching and re- 

 branching ^ and without a central disk ;" and he placed under 

 this head both Pleurograpsus linearis and Graptolithus gracilis 

 (ibid. vol. V. p. 129). Now a, comparison of the respec- 



