of the British Species and Genera of Graptolites. 349 
characterized by the exclusive possession of the genera Dic- 
tyonema, Dichograpsus, Tetragrapsus, Dendrograpsus, Phyllo- 
grapsus, Pleurograpsus, Helicograpsus, Cyrtograpsus, Callo- 
grapsus, Diplograpsus, Climacograpsus, Dicranograpsus, Di- 
dymograpsus, and Rastrites. Of the remaining British genera, 
Graptolites and Retiolites are common to both the lower and 
upper divisions of the Silurian series. The same is really the 
case with Ptilograpsus, for, though not known to occur in 
Britain out of the Lower Ludlow rocks, it is a characteristic 
genus in the Quebec group of Canada. It may therefore be 
said that the Upper Silurian rocks of Britain are not in the 
exclusive possession of any genus of Graptolites. 
The genus Dictyonema, Hall, occurs in the Tremadoc Slates 
(Uppermost Lingula Flags of Salter, Upper Cambrian of Belt), 
in the Lower Llandeilo, and in the Caradoc group. Though 
represented in America by various species in the Quebec group, 
it has not as yet been found in the corresponding strata in 
Britain, viz. the Skiddaw Slates (lowest Llandeilo). The 
species of the Tremadoc Slates is the familiar D. socéale, Salt.; 
and an undetermined form, possibly the same, is found in the 
Lower Llandeilo rocks of the Shelve district. The Upper 
Llandeilo rocks have not hitherto yielded any example of the 
genus; but a single example (apparently D. gracile, Hall) has 
been discovered by Prof. Harkness in the Bala Limestone of 
Girvan. The younger deposits of Britain have not been shown 
to contain any species of Dictyonema; but such have been 
found in the Upper Silurians of America, and even in the 
Middle Old Red Sandstone (Upper Helderberg and Hamilton 
groups); so that the genus cannot be looked upon as charac- 
teristic of any portion of the Silurian period. 
The genera Dichograpsus, Tetragrapsus, and Phyllograpsus 
are exclusively confined to the Skiddaw and Quebec groups, 
both at home and abroad, being represented by several species 
in the Skiddaw Slates of Cumberland and Westmoreland. 
(See Quart. Journ. Geol. Soc. vol. xix. p. 137, and vol. xxiv. 
p- 125 et seq.). Phyllograpsus, represented by its two most 
typical species, viz. P. angustifolius and P. typus, appears to 
range throughout the entire group; but Dichograpsus and 
Tetragrapsus, with four species each, seem to be confined to 
the lower beds of the series. No member of these three genera 
has hitherto been found in the equivalent strata in Wales 
(Whitesand Bay). 
The genus Dendrograpsus, Hall, is mostly characteristic of 
the base of the Lower Silurian series. One species, viz. D. 
Hallianus, Prout, has been dubiously determined from the 
Skiddaw Slates; and D. furcatula, Salt., from the Lower 
