208 T. RUDDY ON THE CAMBRIAN AND SILURIAN IN NORTH WALES. 
of Ptilodictya explanata takes their place. In this bed Orthisina 
first occurs. Here we find also Orthis crispa, O. flabellulum, and 
Strophomena grandis plentifully ; Trilobites, Modiolopsis, a species 
resembling an Ambonychia, Bellerophon resembling perturbatus, and 
a few well-preserved specimens of Orthis spiriferoides (not usually 
‘found so high in the Bala beds). In this zone also are several 
species of Petraia, and a few other undetermined fossils. 
The zone No. 6 can hardly be separated from that below it. In 
it we find Orthisina and Petraca; but in it also we suddenly come 
upon great abundance of Leptene, especially L. transversalis, which 
is common in No. 8. No. 5 consists of hard compact grits from 
15 to 20 feet in thickness, of the same lithological character as the 
Hirnant grit, and occupying the same stratigraphical position; but 
as I have only found one or two small and badly preserved shells in 
it (except near the base, where the line between it and the under- 
lying beds was not clear), I can give no fossil evidence; but the 
sections seem to prove that they are the equivalent of the Hirnant 
grits. 
