SILURIAN ROCKS OF NORTH WALES. 159 
The shell I have referred to as Meristella crassa in these lists 
seems to be absolutely confined to the basement bed. What 
I would suggest a doubt about is the identification of this 
clean-cut form with the coarser and more irregular undulating 
casts seen so commonly at Cefn Rhyddan for instance, near 
Llandovery. 
The northern variety occurs at Llandovery, but I have not 
found the rougher Llandovery variety in the north. Yet it does 
not seem likely that we have here a species more allied to 
M. augustifrons, or the still higher JZ. sumida. This is a point 
of considerable importance in stratigraphy, and I would invite 
the attention of specialists to it. 
Another troublesome shell is Orthis biforata which seems to 
run through from Bala to May Hill. Some occurrences, in 
Silurian, recorded on the evidence of fragments, turned out to be 
founded on OQ. insudaris; some probably on Rhynchonella. This 
shell requires re-examination—especially as we hardly ever 
find more than a cast, and collectors are not careful enough 
about preserving both the outside and inside cast of the very 
same specimen. 
Then we have doubtful forms in Orthis hirnantensis from the 
basement beds which it is hard to distinguish in bad specimens 
from Strophomena silurizna, an Upper Bala fossil. In like 
manner imperfect specimens of Orthis spiriferoides of the Bala 
may suggest the occurrence of Orthis biforata. 
The characteristic fossil of the Silurian Rocks is Cardiola 
interrupta. As far as I know it runs through the whole system 
from the Ludlow Rocks to the bottom of the Denbigh and 
Coniston Flags. It is common in the uppermost beds, and I 
have found it in the Sedbergh District at the very base of the 
flags. Moreover, it has not yet been found above or. below the 
Silurian. The statement of Barranpe that it occurred in the 
Bala Series in this country was founded on an error. It is 
abundant in the Coniston Grits, which were at one time 
supposed by Murcutson and others to be the equivalent of the 
Caradoc Sandstone. ‘Therefore, Cardiola interrupta got into the 
lists of Caradoc Sandstone Fossils. When, however, this 
mistake was corrected and the Coniston Grits were shown to be 
of Silurian age, the necessary correction was not made in the 
fossil lists, and Cardiola interrupia was left in the list of 
Caradoc Fossils from which BarranpE took it. 
From various horizons in the Graptolithic Mudstones of 
North Wales it is very rare to find any fossils but Graptolites. 
1 have, however, frequently found Lingula brevis in them in 
Spengill, and in the Sedbergh district; and in the Spengill 
Limestone (see pp. 142, 148) there is a small species allied to 
Leplana quinguecostata, perhaps only a young form. We 
should look out for these in North Wales. 
