of the British Species and Genera of Grajptolites. 349 



characterized by the exclusive possession of the genera Die- 

 tyonemaj DichograpsuSj Tetragrapsus^ Dendrograpsus^ Phyllo- 

 grapsuSj Pleurograpsus^ Helicograpsus^ Cyrtograpsus^ Callo- 

 grapsuSj Diplograpsus^ CUmacograpsus^ 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 reallj 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. sociale^ 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 Bed 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. 

 ^.126 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. 

 Hallianusj Prout, has been dubiously determined from the 

 Skiddaw Slates j and D. furcatula^ Salt., from the Lower 



