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 Graptolithus, 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 Graptolites as 
G. sagittarius, Linn., G. Sedgwickit, Portl., &e. 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 Oladograpsus 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 Déidymograpsi) 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. 11. 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 Plewro- 
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 Cladograpsus, redefining it as fol- 
lows :—‘‘ Polypary compound, growing bilaterally from the 
primary point, irregularly and repeatedly branching and re- 
branching, and without a central disk ;” and he placed under 
this head both Plewrograpsus linearis and Graptolithus gracilis 
(ibid. vol. v. p. 129). Now a comparison of the respec- 
