SPINE-TAILS. 
149 
i close together; the caudal is large, forked, or 
j crescent-shaped ; the upper division sometimes 
j more developed than the lower ; the head is ob- 
jl tuse and high, the profile approaching to vertical. 
The fishes of this genus are reckoned among 
that small number of the Class which feed en- 
tirely on vegetable substances: sea-weeds, and 
I such like marine vegetation, that may generally 
be found fringing the submerged rocks in the 
tropical seas, afford them an always abundant 
I repast. One result of this diet is, that the 
flesh, though commonly eaten, has a peculiar 
flavour, disagreeable to many persons. The in- 
' testinal canal, as usual in herbivorous animals, 
, is long and complicated. 
' We must consider the caudal lancets of these 
I fishes as defensive, rather than offensive, weapons, 
I analogous to the horns of the ruminant Mam- 
malia. They are highly curious; each consists 
of a curved, flattened spine, lodged in a mem- 
branous sheath in the side of the tail, and ordi- 
narily concealed ; but capable of being partially 
elevated, as on a hinge, at the will of the animal, 
when its acute point and keen edge are found 
1 to point backward. They are exceedingly firm 
j in texture, of a hard crystalline substance ; and 
being violently jerked from side to side, by the 
action of the powerful lateral muscles, they doubt- 
j less constitute formidable weapons, and enable 
the browsing fish effectively to repel any car- 
i nivorous foe that may be inclined to attack him 
in the rear. 
We shall illustrate the genus by a species 
which the inhabitants of Jamaica distinguish as 
the White Doctor-fish [AcantJmrus cceruleus^ 
