196 Mr. C. Lapworth on the Geological 



(vi.) Descending to the species of Rhabdophora we find that 

 instead of ranging through enormous thicknesses of rock, as 

 hitherto supposed, they are so restricted in vertical distribution 

 that few have a more extended range than that which is covered 

 by a single formation in the vertical series ; while the vast 

 majority are peculiar to a single subformation, or mark certain 

 special horizons outside of which they are wholly unknown. 

 As might have been anticipated, the forms which have the 

 greatest longevity present us with the greatest number of 

 recognizable varieties, while the species of shorter range rarely 

 show any notable departure from the primitive type. 



(vii.) The ascertained restriction of the divisions, families, 

 and genera of the Rhabdophora in time necessarily gives to 

 the collective Graptolitic fauna of each of the subsystems or 

 major formations of the Lower Palaeozoic rocks a special and 

 distinctive aspect that renders it capable of immediate identifi- 

 cation all over the world. The Arenig division is recognizable 

 at a glance by its crowds of Phyllograptidge andDichograptidge; 

 the Bala by the absence of these families and the presence of 

 multitudes of Dicellograpta and Diplograptidee ; the Valentian 

 by the absence of the former and the presence of the latter in asso- 

 ciation with Monograptidge ; and the higher Silurians by the 

 absence of the Diplograptidee and the presence of Mono- 

 graptidse alone. 



(viii.) The further restriction in time and vertical extension 

 of the species and varieties of the Rhabclophora places in our 

 hands the material available for a more minute subdivision of 

 the formations of the Lower Palaeozoic rocks than has hitherto 

 been attempted. These subdivisions or Graptolite horizons 

 answer roughly to the Ammonite zones of the Jurassic rocks 

 of Europe, and will, in all probability, prove of equal value 

 in the correlation of widely separated deposits. At present 

 the following zones are recognizable, many of them of extra- 

 ordinary geographical range : — 



Upper Cambrian. 



1. Zone of Bryograptus Callavei, Lapw. — In the Upper 

 Cambrian we know as yet only a single Graptolitic zone, that 

 of the Olenus-heds of Scania and their extra-Scanian repre- 

 sentatives. It may be termed the zone of Dichograptus (?) tenel- 

 lusj Linnrs., or Bryograptus Callavei, Lapw. As I have 

 already indicated, we are almost totally ignorant of the details 

 of its Graptolitic fauna. It seems to be characterized, how- 

 ever, by similar forms of Dichograptidse in Scania, Norway, 

 Shropshire, and the Malvern Hills. 



