HORSE-MACKERELS. 363 



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 CAEANGID^. 



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 



