126 TI'E OLD RED SANDSTONE. 
ing a gap in the record. Three distinct formations the group 
undoubtedly contains — perhaps more; nor will the fact 
appear strange to the reader who remembers how numerous 
the formations are that lie over and under it, and that its vast 
depth of ten thousand feet equals that of the whole secondary 
system from top to bottom. Eight such formations as the 
Oolite, or ten such formations as the Chalk, could rest, the 
one over the other, in the space occupied by a group so 
enormous. ‘To the evidence of its three distant formations, 
which is of a very simple character, 1 shall advert as I go 
along. 
The central or Cornstone division of the system in Eng- 
land is characterized throughout its vast depth by a peculiar 
family of ichthyolites, which occur in none of the other 
divisions. I have already had occasion to refer to the Cepha- 
laspis. Four species of this fish have been discovered in 
the Cornstones of Hereford, Salop, Worcester, Monmouth, and 
Brecon ;* ‘ and as they are always found,” says Mr. Murch- 
ison, “in the same division of the Old Red System, they 
have become valuable auxiliaries in enabling the geologist to 
identify its subdivisions through England and Wales, and also 
to institute direct comparisons between the different strata of 
the Old Red Sandstone of England and Scotland.”+ The 
Cephalaspis is one of the most curious ichthyolites of the sys- 
tem. (See Plate XIII., fig.1.) Has the reader ever seen a 
saddler’s cutting knife?—a tool with a crescent-shaped 
blade, and the handle fixed transversely in the centre of its 
concave side. In general outline the Cephalaspis resembled 
this tool — the crescent-shaped blade representing the head 
the transverse handle the body. We have but to give the 
* Cephalaspis Lewisii, C. Lloydii, C. Lyellii, and C. rostratus 
+ See Note E 2. 
