GREGORY: FISH SKULLS 



281 



Zanclus (Fig. 158), which is usually classed as a chaetodontoid, really belongs with Acan- 

 thurus (Hepatus) and its relatives. This seems at first sight to strengthen Boulenger's 

 conclusion. But a comparative study of the skulls suggests that it is only the stem forms 

 of the chsetodonts, perhaps most nearly represented today by Platax, which can claim a 



Zanclus cornutus 



Fig. 158. Zanclus cornutus. 



remote relationship with the acanthurids, zanclids and balistids, and that the cleft between 

 the typical chaetodonts and the acanthurids is much greater than that between the latter 

 and the plectognath stem as represented by the balistids. 



The chsetodonts and their relatives are referred by Tate Regan (1913a, p. 112; 1929, 

 pp. 320-323) to the perciform division of the suborder Percoidea of the order Percomorphi. 

 The acanthurids and their allies form another suborder, Teuthidoidea, of the same order, 

 while the balistids are classed with the puffers, etc., as a separate order, Plectognathi. 

 The reference of the teuthidoids to the order Percomorphi rests chiefly upon their retention 

 of several primitive percomorph characters in the fins and vertebrae, while the separation 

 of the balistids is due to their loss of primitive characters. Although the plectognaths 

 have acquired conspicuous specializations, which have masked their relationships with the 

 teuthids, we may nevertheless assign them to a new group called the Balistoidei, including 

 the zanclids, the teuthidoids and the plectognaths. In spite of their diverse specializations 

 in the fins and body-form, the zanclids, teuthidoids and balistids possess a common heritage 

 in their skull type, which is already curiously specialized even in the teuthids (Fig. 159), 

 on the whole the least advanced members of the series. 



We have seen in the chaetodonts and many earlier small-mouthed forms how the suspen- 



