1879,] of life forms to breaks of continuity in the strata. 251 



Conocoryphe has representative species in the Menevian, and 

 allied forms appear in the Lingula Flags, Tremadoc and even 

 Lower Bala Beds. Though, it may be that the varieties with 

 the more pronounced glabella, and indeed all the later forms 

 may be separated from the typical genus, for our enquiry the 

 name matters not. All allow that they are allied form's. 



We might have taken Microdiscus and Agnostus instead of Para- 

 doxides and Conocoryphe with similar result. Now except perhaps 

 at the base of the Arenig, no one holds that there is any im- 

 portant break in the succession of strata over the area from which 

 these forms have been procured. They do not appear immediately 

 after an unconformity; they do not disappear just before one. 



It is important to dwell upon- the groups which appear in 

 the earliest rocks yet discovered in Britain, for we shall see that 

 so many forms of life are represented, and they range through 

 subsequent periods to such varying lengths of time that there 

 is nothing to suggest a different state of temperature, atmosphere, 

 or other circumstances, which our recent experience tells us 

 principally affect life. 



The bivalve Crustacea such as Leperditia are few and far 

 between, and there is still less use calling in as evidence worm 

 tracks which seem common to all periods or ill-understood fossils 

 such as Oldhamia or the later Cruziana or obscure sponges. 

 It is, however, very important to notice that in Theca we have 

 the Pteropoda represented, and that although the small differences 

 in that not very complex fossil have enabled palgsontologists to 

 assign different specific names to those which are found in almost 

 every distinct horizon, and even to cut off Stenotheca and Cyrto- 

 theca of the Menevian under different generic names, there is no 

 unconformity between the horizons at which they occur and we 

 follow the genus up into the upper beds of the Silurian without 

 great variation of form ; but of course free swimming oceanic crea- 

 tures would be seldom affected by local changes. 



Brachiopods appear among the first, being represented by the 

 genera Discina, Obolella and Lingulella. The last which used 

 to be called Lingula reaches its' greatest numerical development 

 in the next conformably succeeding group, named from the preva- 

 lence of this shell the Lingula Flags, and is represented by a 

 closely allied form L. anatina, at the present time; so -it at any 

 rate has tided across a good many unconformities. 



To move a little higher in the strata and watch new forms 

 appearing. The Menevian is bracketed with the preceding 

 group. There is no break between them; the general fades 

 of the fossils is the same, some forms are identical. Yet here, 

 and as far as discovery has gone, no lower, we get the trilobites, 

 Arionellus, Anopolenus, Erinnys, Holocephalina. 



