HORSE-MA CKERELS. 
3 6 3 
The first family is typified by a genus (Acronurus) the representatives 
of which are popularly known by the name of “ surgeons,” owing to the presence 
of a sharp lancet-like spine on each side of the tail in the adult. In addition 
to the presence of one or more such spines or bony plates, the family is further 
characterised by a single dorsal fin, with a very small number of spines. The 
body is compressed, and oblong or deep in form, with a covering of minute 
scales; the moderate-sized eyes are lateral in position; the small mouth is 
furnished in front with a single series of more or less compressed upper and 
lower incisors, which may be either pointed or serrated; but the palate is toothless. 
The pelvic fins are thoracic in position, and the hinder extremity of the air-bladder 
is forked. These fishes are inhabitants of all the tropical seas, and are most 
common in the neighbourhood of coral-reefs and islands, where some feed on the 
soft polyps of the coral, but others on various vegetable substances. In the true 
surgeons ( Acronurus) there is an erectile spine situated in a groove on each side 
of the tail; and the pelvic fins are generally furnished with a single spine and 
five rays. In the young the body is scaleless, and the tail-spines either very small 
or wanting. These fishes are represented by a large number of species, the largest 
of which does not exceed 18 inches in length, and they are distributed over all 
tropical seas with the exception of the Eastern Pacific. In a fossil state the genus, 
like the next, occurs in the middle Eocene beds of Monte Bolca, in Italy. From 
the true surgeons the members of the genus Naseus, which range over the Tropical 
Pacific and the Indian Ocean, are distinguished by having from one to three non- 
erectile spines on each side of the tail, and the presence of only three rays in the 
ventral fins; while in some forms the head is armed with a forwardly-directed 
bony horn or crest-like prominence. The minute and rough scales make the skin 
like fine shagreen. A third genus ( Prionurus ) differs in having a series of keeled 
bony plates instead of spines on the sides of the tail. All the species of Naseus 
are said to be purely herbivorous. The true surgeons use their spines as formid¬ 
able weapons of attack by erecting them and striking sideways with their tails. 
The Horse-Mackerels, —Family CarangidsE. 
Although the name horse-mackerel properly applies only to a single British 
fish ( Caranx trachurus), otherwise known as the scad, it may conveniently be 
made to do duty for the whole of the members of the family to which that species 
pertains. Having the body more or less compressed, these fishes are specially 
distinguished by the teeth, when present, being villiform or conical. The spinous 
portion of the dorsal fin is sometimes rudimental; the hinder rays of both the 
dorsal and anal may be broken up into separate finlets; and, when present, the 
pelvic fins are thoracic in position. In the skeleton there are ten trunk, 
and fourteen tail vertebrae; although in one genus the number of the latter is 
increased to sixteen. The gill-openings are wide, the eyes lateral, and there is no 
bony stay connecting the preopercular with the infraorbital ring. The scales, 
which are usually small, may be altogether wanting; and in many cases the lateral 
line is wholly or partially armed with shield-like overlapping plates. There is 
always an air-bladder. In the young of some forms there is an armature of the 
