160 SILURIAN ROCKS OF NORTH WALES. 
It would also appear desirable to ask paleontologists to give 
us a clearer definition of the forms of Favosz/es which we refer to 
under the specific names, aspera, alveolaris, and multipora; and 
to reconcile or correct the discrepancies in the descriptions of 
these species given by different authors It will probably be 
found that the common form in our basement bed can be 
distinguished from those which range far down into the under- 
lying series. 
What I have endeavoured to bring out in the foregoing 
sketch is, that there is a sequence of rocks in North Wales to 
be made out by an examination of a large number of sections 
which finds a strict parallel in the North of England, and 
can be in more general terms compared with the Silurian 
in South Wales ‘That there are certain lithological and 
pakeontological zones at which the correlation is exact, 
suggesting that what is found in other areas between those 
horizons may be profitably sought for. [1 have not collated all 
the valuable work in the details of the series chiefly referred to 
which has been done by others since the reading of this paper, 
but offer it as a contribution in the same direction, hoping that 
it may be found useful in aid of future work. | 
