SPINE-TAILS. 149 



close together; the caudal is large, forked, or 

 crescent-shaped ; the upper division sometimes 

 more developed than the lower ; the head is ob- 

 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 

 such like marine vegetation, that may generally 

 be found fringing the submerged rocks in the 

 tropical seas, afford them an always abundant 

 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 

 fishes as defensive, rather than offensive, weapons, 

 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 

 to point backward. They are exceedingly firm 

 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- 

 less constitute formidable weapons, and enable 

 the browsing fish effectively to repel any car- 

 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 (Acanthurus ccsruleus, 



