250 Prof. Hughes, On the relation of the appearance [May 5, 



There were local checks in the movements as might be expected, 

 especially in such a long time, but, on the whole, conditions were 

 very similar over our area, and indeed very far beyond it during 

 that enormous period of depression. The Silurian has at its base 

 a stronger break than occurs between any of the subordinate 

 divisions of the Cambrian Period, but still there is no great 

 interval made out between the Silurian and Cambrian and they 

 might with advantage be bracketed together, and take in also 

 some beds which have passed as Old Red, and be all called by 

 one of the old names. Transition Rocks or Greywacke Group. 



Then comes a great interruption and waste of lands before 

 the submergence which gave us the Upper Old Red conglomerates, 

 the Devonian Rocks, and the Carboniferous, all, I take it, to be 

 bracketed together as belonging to one set of conditions. 



Then another great interval, and up and down over an enor- 

 mous area. As we get on in the world's history, in Carboniferous 

 times to some extent, but more in Mercian* times, and still more 

 in the Anglian series, we find the deposits indicating distinct hydro- 

 graphical areas, and this must influence the distribution of life. 



Now with regard to the first ht [ ^jearance of life in Britain, 

 there are some very curious facts tdi" be noticed. In the earliest 

 fossiliferous rocks, we have net anything like a common rudi- 

 mentary form ; but a large nuinber of different families are re- 

 presented, and represented by unany genera and species. 



They have turned up it different horizons, some low down, 

 some higher up, but not insrich a way as to suggest any grouping 

 or order of succession, bu '^ rather to make it certain that it is 

 only from our not having been able to find the remains preserved, 

 that we do not get them abundantly all through, for they must 

 have existed throughout the period. 



Not at the base of the Harlech group, but where red slates 

 come in, showing a local difference in the character of the sedi- 

 ment, we find a lingulella and several trilobites. Others come in 

 at different horizons all the way up. Take for example the Trilo- 

 bites Conocoryphe, Plutonia, Paradoxides, which occur down in the 

 lowest beds. They appear, with many other forms, fully specialized 

 and well developed. Just let us follow these up. Plutonia disappears 

 at once ; Paradoxides has its representative species in the Mene- 

 vian, the next overlying group, and the genus then disappears ; 



* This term Mercian I use for all the deposits from the Lower New Ked or Per- 

 mian up to the top of the Jurassic Series, leaving in doubt for the present whether 

 some of the estuarian and freshwater beds which show a sUting up of the basin and 

 emergence at the close of the period should be bracketed with the Jurassic or form 

 the base of a new series. Under the name Anglian I include Neocomian and Cre- 

 taceous, as the term Cretaceous has become somewhat unsettled. At any rate we 

 are safe in commencing a distinct group with the Lower Greensand, and not at- 

 taching great importance to the break which seems probably to occur in some 

 places at the base of the Upper Greensand. 



