Distribution of the Rhabdophora. 277 



The grand characteristic of the graptolitic fauna of the 

 Arenig rocks is the extraordinary predominance of those forms 

 of Rhabdophora which compose the family of the Dicho- 

 graptidaa. In every region where these Arenig strata have 

 been fully investigated they have been found to be locally 

 crowded with the feathery forms of this special group, from 

 the simplest to the most highly complex species. Of the 

 sixty-three forms of Graptolites enumerated in the foregoing 

 Table, forty-five, or nearly three fourths of the entire fauna, 

 belong to this single family; and if we unite with it the 

 dubiously distinct family of the Phyllograptidas, it will be 

 found to outnumber the representatives of all the remaining 

 families in the proportion of nearly five to one. This marked 

 peculiarity, which gives to our Arenig fauna a special cha- 

 racter unique among those of the Lower Paleozoic forma- 

 tions, is not confined to Britain alone, but is fully as charac- 

 teristic of the Arenig of Scandinavia, and of North America, 

 as well as of the antipodal regions of Australia. 



Within the generally accepted provisional limits of the 

 Arenig formation there is some evidence of a progressive 

 change in the relative preponderance of the various genera of 

 the Dichograptidaa in proportion as we ascend in the vertical 

 series. As we have already pointed out, the more complex 

 genera seem to have been the first to appear ; and in the lowest 

 Arenig beds they are still abundant, but they are intermixed 

 with simple forms. In the Middle Arenig strata the highly 

 complex and beautifully regular genera like Clonograptus and 

 Dichograptus appear to have died out, and the prevalent com- 

 plex genus in these beds is the four- armed Tetragraptus. In 

 the true Upper Arenig the bifid genus Didymograpfus is 

 supreme. The monotonous character of the fauna of these 

 higher beds, due to the presence of multitudes of examples of 

 this single genus, is but slightly affected by the presence ot 

 scattered examples of irregularly compound genera, very 

 different in their structural features from the regularly dicho- 

 tomizing forms of the earlier beds. 



All the regularly dividing forms of Dichograptidse with 

 more than four branches appear to have vanished before the 

 close of the so-called Middle Arenig. There the four-armed 

 Tetragraptus is most characteristic. The bifid genus Didy- 

 mograptus is rare in the lowest beds ; but its individuals increase 

 rapidly in numbers as we ascend the succession, filling the 

 places left vacant by the disappearing complex genera, till 

 finally in the Upper Arenig it becomes the most prevalent and 

 characteristic form. The rarer and irregularly branching 

 complex genera which there accompany it seem to point to- 



