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



species are counted; all the most complex forms are here with 

 single rows of cells, double rows, four rows back to back ; grapto- 

 lites branching once or many times, symmetrically or irregularly, 

 all are represented*. Such a full complement of variously de- 

 veloped forms does certainly make one suspect that the group 

 will be found in older beds, probably as low down as the base 

 ■of the Cambrian at least, wherever suitable conditions pre- 

 vailed. 



The many branched forms soon disappear and the twin 

 Graptolites do not get above the Lower Bala Beds. There is 

 certainly no unconformity there. The Diprionidian forms survive 

 the break at the top of the Cambrian, and die out in the lowest 

 beds of the Silurian in Britain as in Bohemia. The apparent 

 exception Retiolites which runs much higher belongs to a distinct 

 group. Before we get to the top of the Silurian they have all 

 completely gone. 



We might take almost any of the genera or species of Arenig 

 trilobites, and we should, in the same way tracing them on, find 

 that some dropped out sooner and some later, but that they in no 

 marked way ended their appearance at a recognised physical break, 

 except perhaps the genus Trinucleus, which has not yet been 

 shown to have got back into our area after the interval between 

 the Cambrian and Silurian. 



Lamellibranchs in the Arenig, represented by Palaearca and 

 Ctenodonta, form a more conspicuous group in the Bala Beds', and 

 increase in importance up to the top of the Silurian. For it 

 must be noticed that although there may be a larger number of 

 species of lamellibranchs in the Wenlock, the species of the 

 Ludlow Rocks bear a much larger proportion to the rest of the 

 life of that period. 



Another group must be noticed though not so suitable for our 

 purpose. Corals are not common except in the limestones, which 

 of course they have largely helped to form. So Corals come in 

 with the Limestones of Llandeilo and Bala, and as at present 

 arranged there is hardly a genus which does not cross the gap be- 

 tween Cambrian and Silurian, and turn up again in the Wenlock 

 Limestone; but here as well as in the case of the Echinoderms, 

 we must remember the richness of the Wenlock in other fossils 

 and their wonderful state of preservation, and also the ease 

 with which they can be obtained. 



Cystideans appeared early though few and far between, from 

 the Protocystites of the Menevian to the Echiuosphaerites and 

 Sphaeronites of the Bala. ' Encrinite stems occur sometimes 



* There was a suspicion of an allied form in the older rocks in Dictyonema 

 sociale, and Mr Clifton Ward believes that beds from which he has obtained 

 grajitolites in the Lake district are of Tremadoc age. 



