ANGEL-FISHES. 157 



dorsal spine greatly elongated and thread-like ; the colours vary 

 in different specimens, although the arrangement of the light and 

 dark bands is much the same in all (compare 6.23 and 624). 

 These tishes are among the commonest in the tropical Indo-Pacific 

 area. 



The species of Holacanthus are known generally as Angel-fishes, Angel- 

 the name being specially applied to Holacanthus ciliaris, 629; 

 they are larger than those of Chcetodon and not less bright in the 

 coloration ; they have a stout spine projecting backward from 

 the preopercular bone which is wanting in Chcetodon. They are 

 all esteemed as food, particularly the Holacanthus imperator (627), 

 which attains a length of 15 inches. 



The genus Pomacanthus includes American tishes only ; they 

 are larger than the species of Holacanthus, and have about 10 

 spines in the dorsal tin instead of 14. The young are brightly 

 coloured, frequently with yellow bands, and they differ among 

 themselves considerably in their coloration ; the adults are blackish. 

 The Paru, Pomacanthus para, 632, is a well-known fish of the 

 West Indies. 



In Scatophagies (633) and Ephippus (635) there is more 

 distinction between the spinous and soft portions of the dorsal fin 

 than in the majority of the Chsetodonts, owing to the presence of 

 a fairly deep notch, and the scales do not spread over the spinous 

 portion; the preopercular spine is wanting. Scatophagus argus 

 (633) is a common Indian shore tish, and is sometimes caught in 

 the rivers. Old specimens of an Atlantic species of Ephippus Ephippus. 

 (Ephippus faber, 635) show almost constantly bulbous enlarge- 

 ments of the frontal and supraoccipital bones (636), and sometimes 

 also of the foremost neural spines. These swellings have been 

 attributed to a diseased condition of the bones ; whether or no 

 this is the correct explanation, the right and left symmetry of the 

 swellings, the constancy of their occurrence in old fishes, and the 

 limitation of them to the bones above mentioned are features of 

 interest. 



The family Drepanidse contains a single species, Drepane 

 punctata, 637, occurring in the Indian Ocean and North Australian 

 seas, closely related to the Chsetodontidas^but distinguishable by 

 the very long, curved pectoral fins. 



