98 A HISTORY OF FISHES 



replaced by small, movable spines, which stand erect when the 

 body is inflated with air. The Porcupine-fishes and Burr-fishes 

 [Diodontidae) have an even stronger protection, the roots of the 

 long, stout spines coming into contact with one another and 

 providing a more or less continuous coat of mail. In some 

 species these spines are two-rooted and movable, so that they 

 can be laid back flat or erected at will; in other forms they 

 are three-rooted and fixed (Fig. 420). 



The Lung-fishes {Dipneusti) are the sole living representatives 

 of a once large and important group of fishes, in which the 

 scales have a structure different to those of all other Bony 

 Fishes. The primitive members of this group had a covering 

 of ganoid scales, which must have been derived from plates 

 similar to those of the Palaeoniscids, but the cosmine layer has 

 a special regular structure and the ganoine is reduced to a thin 

 superficial covering. Of the existing forms, the Australian genus 

 (Epiceratodus) has large, thin, overlapping cycloid scales, whereas 

 in the African {Protopterus) and South American [Lepidosiren) 

 forms the scales are very much smaller, completely embedded in 

 the skin, and more or less separated from one another (Fig. 99). 



With few exceptions, the scales of Bony Fishes have a regular 

 arrangement, and within certain limits both their size and dis- 

 position is constant for any given species. For this reason, a 

 count of the number of scales is often of some importance in 

 identifying any particular fish. Generally, the scales are 

 arranged in obliquely transverse series, and the number of 

 these series is counted along the middle of the side from behind 

 the gill-opening to the base of the caudal fin. In estimating the 

 number of scales across the body {i.e. the number of longitudinal 

 rows) they are usually counted in one of the transverse series, 

 as a rule that which runs from the commencement of the 

 dorsal fin downwards and forwards to the lateral line, and 

 from thence downwards and backwards to the pelvic fin (Fig. 

 43). Thus the scale formula for a particular species may be 



6-7 

 written : 44-47 — '— . This means that there are from 44 to 47 

 9-10 



scales in a longitudinal series from the head to the tail, 6 or 7 

 between the origin of the dorsal fin and the lateral line, and 

 9 or 10 between the latter and the base of the pelvic fin. The 

 Salmon {Salmo salar) and Trout [S. trutta) of our own country 

 provide an example of the importance of scale-counts in dis- 

 tinguishing closely related species. These fishes are extremely 

 similar in most characters, and although there are a number of 



