618 The Squamipinnes 
Mexico, and is generally common, though rarely entering rock 
pools. 
Zanclus eocenus is recorded from the Italian Eocene. 
The Tangs: Acanthuride.—In the next family, Acanthuride, 
the surgeon-fishes or tangs, the scales remain small and shagreen- 
like, the body is more elongate, the gill-openings still more 
restricted, and the teeth are flattened and incisor-like. The pubic 
bone is more elongate, and in all the species some sort of arma- 
ture is developed on the side of the tail. The spinous dorsal 
Fig. 512.—Teuthis ceruleus (Bloch & Schneider), Blue Tang; Medico. 
Family Teuthidide. 
in all is less developed than the soft dorsal. The species abound 
in the warm seas, especially about the tide pools, and are used 
as food. They undergo considerable changes. with age, the 
caudal armature being developed by degrees. Nearly all are 
dull brown in color, but in some a vivid ornamentation is added. 
Fossil forms are found from the Eocene and later. Most of 
these are referable to Teuthis and Acanthurus. 
The principal genus is Teuilis, characterized by the presence 
on each side of the tail of a sharp, knife-like, movable spine 
with the point turned forwards and dropping into a sheath. 
This spine gives these fishes their name of surgeon-fish, doctor- 
