28 On the Geological Distribution of the Rhabdophora. 



would be confined to the Llandeilo and Bala periods, Azygo- 

 graptus being the only genus of this family known in the 

 Arenig formation. 



Leptograptus &c. — Turning next to the second division of 

 the family, we come to the type genus Leptograptus, one of 

 the most puzzling of all the monoprionidian Grraptolites. So 

 frequently does it throw off extra simple or compound branches 

 both from the arms and sicula, that there is some suspicion 

 that the so-called genera Pleurograptus, Amphigraptus, &c, as 

 defined by myself, are merely names for permanent varieties 

 of this form. A few examples of Leptograptus have been 

 met with in the Middle Llandeilo ; but examples of this genus 

 are not common till we pass above the line of the Bala Lime- 

 stone, where, in the south of Scotland, they are found in 

 extraordinary abundance. 



The genera Pleurograptus and Amphigraptus are Upper 

 Bala forms exclusively, occurring in association with Lepto- 

 graptus in the Lower Hartfell shales in amazing profusion. 



The only species of this group deserving special mention 

 here are Leptograptus jlaccidus, Hall, and Amphigraptus 

 divergens, Hall. The former is both a Glenkiln and a Hart- 

 fell fossil in Britain and America ; the latter is restricted to 

 the Middle Hartfell beds and the corresponding Lorraine 

 shales of New York. 



Family iii. Dicranograptidse. 



The family of the Dicranograptidge, which is most inti- 

 mately allied zoologically to that of the Leptograptidse, is 

 almost precisely coincident with that family in its vertical 

 range, its oldest known forms occurring in the Upper Arenig, 

 and its newest in the Upper Bala formation. Like the Lepto- 

 graptidas also its species are characteristic of the Bala forma- 

 tion. Few forms are found in the Lower Llandeilo or in the 

 Upper Bala ; but in the Llandeilo-Bala or Glenkiln zone 

 they are, next to the Diplograptidse, by far the commonest 

 fossils. The family probably culminates here ; but though 

 there is a falling off in species there is no decrease in the 

 number of individuals of Dicranograptida? till we reach the 

 Bala horizon of the limestone. Very few forms pass higher 

 in the succession. A few forms are known both in Scotland 

 and South Sweden, in the highest Bala zones, close to the 

 line marking the base of the Silurian, but no Dicranograptidse 

 have hitherto been detected above that limit. 



Dicellograptus. — Of the two genera which make up this 

 family the genus Dicellograptus, Hopk., is the more prolific 

 in species, and seems to have the more extended vertical and 



