12 THE OLD RED SANDSTONE. 
opening, shielded by an operculum, or gill-cover. In the 
Osteolepis the mouth opened below the snout, but not so far 
below it as in the purely cartilaginous fishes— not farther 
below it than in many of the osseous ones—than in the 
genus Aspro, for instance, or than in the genus Polynemus, 
or in even the haddock or cod. It was thickly furnished with 
slender and sharply-pointed teeth. I have hitherto been una- 
ble fully to determine whether, like the mouths of the osseous 
fishes, it was movable on both sides ; though, from the perfect 
form of what seems to be the intermaxillary bone, I cannot 
avoid thinking it was. The gills opened, as in the osseous 
fishes, in continuous lines, and were covered by large bony 
opercules — that on the enamelled side somewhat resemble 
round japanned shields. (See Note D.) 
But while the head of the Osteolepis, with its appendages, 
thus resembled, in some points, the heads of the bony fishes, 
the tail, like those of most of its contemporaries, differed in 
no respect from the tails of cartilaginous ones, such as the 
sturgeon. ‘The vertebral column seems to have run on to 
well nigh the extremity of the caudal fin, which we find de- 
veloped chiefly on the under side. The tail was a one-sided 
tail. Take into account with these peculiarities — peculiari- 
‘ties such as the naked skull, jaws, and operculum, the naked 
and thickly-set rays, and the unequally lobed condition of tail 
—a body covered with scales, that glitter like sheets of mica, 
and assume, according to their position, the parallelogram- 
ical, rhomboidal, angular, or polygonal form —a lateral line 
raised, not depressed —a raised bar on the inner or bony 
side of the scales, which, like the doubled-up end of a tile, 
seems to have served the purpose of fastening them in their 
places—a general clustering of alternate fins towards the 
tail— and the tout ensemble must surely impart to the reader 
