of the British Species and Genera of GraptoUtes. 353 



either the Skiddaw Slates or the Lower Llandeilo proper, though 

 its discovery in the former of these may fairly be anticipated. 

 In the Upper Llandeilo rocks of the south of Scotland a single 

 species has been found, apparently R. venosusj Hall ; and a 

 second species, R. perlatus^ Nich., occurs in the Coniston 

 Flags ; but this may possibly turn out to be a large variety of 

 the former. The third and longest-lived British species is R. 

 6^e^'?^^7^^*<2?^ws,Barrande,which occurs plentifully in the Coniston 

 Flags (Caradoc), and has also been found in the Ludlow rocks 

 of the Pentland Hills, near Edinburgh. 



The genus PtUograpsuSy Hall, is known as occurring in 

 Britain by a single species only, P. anglicuSj Nich., which is 

 found in the Lower Ludlow rocks. The two remaining spe- 

 cies of Ptilograpsus occur in the Quebec group in Canada ; 

 but no member of the genus has as yet been discovered in any 

 of the intervening formations. 



Summary, 



Of the above-mentioned British genera of Graptolites, in 

 number seventeen, it will be seen that fourteen are, as far as 

 is yet known, entirely and exclusively confined to the Lower 

 Silurian series, two are common to both the Lower and Upper 

 divisions, and Ptilograpsus alone is confined to the Upper 

 Silurian rocks. As, however, this last-named genus is found 

 in the Quebec group, the Upper-Silurian period cannot be 

 said to possess a single characteristic genus of the family, and 

 it possesses but two peculiar species. 



In the Tremadoc Slates (Upper Cambrian ?) no other genus 

 is known to occm' than Dictyonema^ and this is represented by 

 a single species. The Skiddaw Slates (Lowest Llandeilo) are 

 specially characterized by the exclusive possession of the ge- 

 nera DichograpsuSj Tetragrapsus ^ and Phyllograpsus^ and of 

 the species Diplograpsus antennariusj D.pristiniformis^ Didy- 

 mograpsus nitiduSy D. VfractuSj and D. affinis. The Skiddaw 

 Slates contain altogether eight genera and twenty-four species, 

 of which three genera and thirteen species belong also to the 

 Quebec group. Two species occur also as characteristic fossils 

 in the Lower Llandeilo rocks of Wales and Sweden. Five 

 species are peculiar to the Slates, and the remaining four 

 occur either in the Upper Llandeilo or in the Caradoc groups. 

 Out of nineteen species, not peculiar to the Skiddaw Slates, 

 thirteen, or more than two-thirds, are common to the Quebec 

 group of Canada, a close relationship between the two forma- 

 tions being thus demonstrated. 



The Lower Llandeilo rocks proper are specially charac- 

 Ann. i& Mag. N. Hist. Ser. 4. Vol ii. 25 



