lxiv BRITISH GRAPTOLITES. 



He regards the Graptolites generally as the " primitive stock " from which the 



various existing sections of the living Hydrozoa originally diverged. 



In his fourth paper Nicholson recorded the occurrence of 



Ptilograpsus in rocks of Ludlow age in Britain, and described 

 Nicholson, . . T . *~ 1 



„~ ,, ~ a new species (./ . anglicus) JNicholson agrees with Hall that 



" On the Occurrence i v & / o 



of the G-enus Ptilo- the genus Ptilograpsus is closely related to Plumularia, and 



grapsus in Britain, that it was probably an attached form. He here modifies his 



with Notes on the previous view as to the invariable presence of the axis in all 



Ludlow Graptolites," G rapto lites, and admits that " the axis is not so constantly 



' , ' present as has generally been thought," that it "is certainly 



YO l i absent" in all the Dendroidea, and "probably in other 



families." 



In addition to P. anglicus, Nicholson records several other forms of Graptolites 



from the Ludlow rocks, viz. Gr. priodon, Gr. colon us, and Gr. Nilssoni. 



-.^ This paper was followed in June by a fifth, in which 



Nicholson, Nicholson proposed the new generic name of Helicograpsus 



" On Helicograpsus," for the species Gr. gracilis of Hall. The essential difference, 



• Ann. and Mag. of Nat. according to him, between this and his own genus Pleuro- 



Hist., ser. iv, vol. n. grapsus (Carruther's Gladograpsus) consists in the presence of 



a distinct "funicle" and regular branching in the former ; whereas in the latter 



there is no funicle, and the branching is quite irregular. 



The sixth paper by Nicholson published in 1868 dealt 



, T . , , ' with the " Distribution in time of the British Species and 



Nicholson, L 



" On the Geological Genera of Graptolites." It may be here briefly summarised 

 Distribution of as giving an excellent idea of the general state of opinion on 



Graptolites," 'Ann. this subject at that date, 

 and Mag. of Nat. Hist.,' ^ The Graptolites as a wllole are characteristically Silu- 



rian, and fourteen out of seventeen genera are exclusively con- 

 fined to the Lower Silurian, the Upper Silurian only possessing two peculiar species. 



(2) In the Tremadoc Slates (= Upper Cambrian) Dictyonema occurs. 



(3) To the Lower Llandeilo (Skiddaw Slates) the genera Dichograpsus, 

 Tetragrapsus, and Phyllograptus, etc., are strictly confined. They occur in 

 association with species of Didymogra/psus and Biplograpsus. 



(4) The Upper Llandeilo Rocks (which include all the graptolitic beds of 

 Scotland) contain the genera Biplograpsus, Glimacograpsus, Graptolites, Bastrites, 

 and Dicranograpsus. 



(5) The Caradoc beds do not as a rule yield Graptolites, but in Ireland they 

 afford Diplog. pristis, Didymog. sextans, Helicog. gracilis, Gr. Nilssoni, Gr. Sedgwickii, 

 Callog. elegans, etc. 



(6) In the Lower Llandovery one Graptolite only has been found — Glimacog. 

 teretiusculus, 



