352 Dr. 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 Dendrograjpsus and Dictyonema. 



The genus Cyrtograpsus^ Carr., appears to have a range 

 similar to that of Helicograpsus (viz. Upper Llandeilo and 

 Caradoc). 



The remaining three genera of British Graptolites, namely, 

 RetioliteSj 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 Caradoc 

 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. 

 Sagittarius — 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. Sagittarius ^ Linn., G. Sedgwickii^ 

 Portl., G. lohiferus^ M^Coy, G. tenuis^ Portl., G. Nilssoni^ 

 Barr., 6^.^r^oc?on, 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- 

 latuSj Barr., G. Bohemicus^ Barr., G. discretuSj Nich., and G, 

 colonusy 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 sm'vive into 

 the Upper Silurian period are G. priodon, G. colofiuSj G. Fle- 

 mingiij 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 RetioUtes^ Barr., has hitherto not been found in 



