1879,] of life forms to breaks of continuity in the strata. 253 



Tremadoc, and itself disappearing at once. The same genera 

 but different species of Pteropoda still prevail. So in the Upper 

 Tremadoc the characteristic Angelina appears and is lost. With 

 it we find Asaphus, Ogygia, Cheirurus, 2 genera and 3 species of 

 Phyllopods, and Theca still among the Pteropods, but, in addition 

 to Theca, we have now Bellerophon and Conularia, both of which 

 genera last through long ages of Cambrian and Silurian, and, 

 surviving the great geographical changes at their close, reappear 

 in the Carboniferous. Conularia tides over another almost equally 

 vast revolution, namely, that which preceded the New Red, and is 

 last seen, not at the close of a period, but in the Lias, an early 

 stage of the Jurassic epoch. 



How long did it take to evolv.e the Cephalopoda with their 

 cartilaginous cranium and optic ganglia ? Hitherto we have 

 found none- in rocks older than the Upper Tremadoc. Salter, 

 speaking of Cyrtoceras, remarks that many forms migrated in 

 Cambrian times eastward from America, and are consequently 

 of older date there than in Britain. But few, he adds, follow 

 a reverse order of progression. Here, however, is the most hopeful 

 line of enquiry, to seek in older rocks in other areas for the pro- 

 genitor of the Tremadoc Orthoceras sericeum. The genus does 

 not die out with the close of a period as far as evidence has yet 

 been collected at home or abroad, for well down in the Carboni- 

 ferous we lose it in Britain, and well up in the Hallstadt Beds 

 we fihd it abroad. 



In the next series we have disputed ground, some having 

 bracketed the Arenig with the Lingula Flags and Tremadoc Beds, 

 others having thought them, though continuous with the older 

 rocks, so much more closely connected palseontologically with 

 the overlying series that they have bracketed them with the 

 Bala group, while some believe that there is an unconformity 

 at the base of the Arenig. This opinion is partly founded on the 

 large number of species found in the Arenig and not in the under- 

 lying series, but in this case it is more obvious than usual how 

 valueless are percentages of species in common where there is not 

 a fair representative series in each. In the Woodwardian Museum 

 Catalogue (published 1873), there are 59 species recorded from the 

 Arenig, and only 17 from the Upper Tremadoc; when that was 

 drawn up it was clear we must have had 42 not common to both. 

 In the newer Catalogue just published by the Museum of Practical 

 Geology, there are 97 recorded from Arenig and only 23 from 

 Upper Tremadoc, shewing 74 that must be peculiar to the Arenig. 



However, after making allowance for this, there do seem to 

 be a large number of new forms appearing for the first time in 

 the Arenig beds. To begin, we have here the most characteristic 

 group of Cambrian and Silurian fossils, the graptolites, 17 to 23 



