352 Dy. H. A. Nicholson on the Distribution in Time 
Upper Llandeilo rocks of Scotland, and has been also made 
out in the Caradoc group in Ireland. 
In these latter beds Mr. Baily has likewise determined the 
existence of the genus Callograpsus, Hall, by the single spe- 
cies C. elegans, Hall. This genus, which is otherwise con- 
fined to the Quebec group, is in most respects intermediate 
between Dendrograpsus and Dict ‘yonema. 
The genus Cyrtograpsus, Carr., appears to have a range 
similar to that of Helicograpsus (wiz. Upper Llandeilo and 
Caradoc). 
The remaining three genera of British Graptolites, namely, 
Retiolites, Graptolites, and Ptilograpsus, are common to both 
the upper and lower divisions of the Silurian rocks, though 
the last has not as yet been detected in Britain except in the 
Upper Silurians. 
The genus Graptolites, Linn., doubtfully represented in the 
Lower Llandeilos by a single species, and largely represented 
in the Upper Llandeilos, attains its maximum in the Caradoe 
series (Coniston Flags). At this point most of the species of 
the genus appear to ‘have died out, no more than four passing 
up into younger deposits. With the doubtful exception of G. 
sagittarvus—a determination which may have been founded on 
a fragment of a compound species—no completely satisfactory 
instance is known to me of the occurrence of any member of 
the genus Graptolites either in the Skiddaw Slates or in the 
Lower Llandeilo proper. In the Upper Llandeilo rocks the 
genus is represented by G. sagittardus, Linn., G. Sedqwickit, 
Portl., G. lobiferus, M‘Coy, G. tenuis, Portl., G. Nilssoni, 
Barr., G. priodon, Bronn, and G. fimbriatus, Nich. All these 
forms, however, pass upwards; so that the Upper Llandeilo 
rocks cannot be said to possess any species of this genus pe- 
culiar to themselves. In the Coniston Flags (Caradoc), besides 
all the species above mentioned, there occur also G. turricu- 
latus, Barr., G. Bohemicus, Barr, G. discretus, Nich., and C. 
colonus, Barr., this last passing on, together with G. priodon, 
into the overlying Coniston Grits, which would seem to be 
likewise of Caradoc age. ‘The four species which survive into 
the Upper Silurian period are G. priodon, G. colonus, G. Fle- 
mingt, Salt., and an undetermined form from the Ludlow 
rocks. Of these, G. Flemingii is peculiar to the Wenlock 
formation ; but the first two are found in both the Wenlock 
and Ludlow rocks. Of all the species of the genus, G. priodon 
has the most extensive vertical range, passing from the Upper 
Llandeilo up to the Upper Ludlow formation. 
The genus Retiolites, Barr., has hitherto not been found in 
