116 THE OLD RED SANDSTONE. 
characterizes, says Agassiz, the fish of all the more ancient 
formations. At one certain point in the descending scale 
Nature entirely alters her plan in the formation of the tail. 
All the ichthyolites above are fashioned after one particular 
type — all below after another and different type. The bibli- 
ographer can tell at what periods in the history of letters one 
character ceased to be employed and another came into use. 
Black letter, for instance, in our own country, was scarce 
ever resorted to for purposes of general literature after the 
reign of James VI.; and in manuscript writing the Italian 
hand superseded the Saxon about the close of the seventeenth 
century. Now, is it not truly wonderful to find an analogous 
change of character in that pictorial history of the’ past 
which Geology furnishes? From the first appearance of ver- 
tebrated existences to the middle beds of the New Red Sand- 
stone, 
a space including the Upper Ludlow rocks, the 
Old Red Sandstone in all its members, the Mountain Lime- 
stone, with the Limestone of Burdie- House, the Coal Meas- 
ures, the Lower New Red, and the Magnesian Limestone, — 
we find only the ancient or unequally lobed type of tail. In 
all the formations above, including the Lias, the Oolite, Mid- 
dle, Upper, and Lower, the Wealden, the Green-Sand, the 
Chalk, and the Tertiary, we find only the equally-lobed con- 
dition of tail. And it is more than probable, that, with the 
tail, the character of the skeleton also changed ; that the more 
ancient type characterized, throughout, the semi-cartilaginous 
order of fishes, just as the more modern type characterizes 
the osseous fishes ; and that the upper line of the Magnesian 
Limestone marks the period at which the order became ex- 
tinct. Conjecture lacks footing in grappling with a revolution 
so extensive and so wonderful. Shall I venture to throw out 
a suggestion on the subject, in connection with another 
