CHAPTER XVIII 



TELEOSTOMI : GENERAL CHARACTERS CROSSOPTERYGII— 



CHONDROSTEI HOLOSTEI 



Sub-Class II. Teleostomi. 



In this group of Fishes the primary upper and lower jaws (palato- 

 quadrate and Meckelian cartilages) are supplemented by the 

 addition of certain tooth-bearing membrane bones which form 

 secondary jaws corresponding to the functional jaws of the higher 

 Craniates.^ The chondrocranium and the primary jaws are 

 usually more or less completely ossified by cartilage bones, and 

 there is always a secondary cranium of dermal bones, of which 

 paired parietals and frontals above, and a median vomer and a 

 parasphenoid below, are amongst the most constant. The skull 

 is hyostylic. An operculum covering the gill-clefts and supported 

 by a special opercular skeleton is a constant feature. The verte- 

 bral column is often acentrous, and when centra are present they 

 are invariably arch-centra. There is a well-developed secondary 

 pectoral girdle, connected dorsally with the hinder part of the 

 skull. As a rule the pelvic girdle is absent altogether, and 

 when present it is rarely more than a rudiment or a vestige. 

 The endoskeletal supports of the paired fins are uniserial. The 

 dermal fin-rays of the paired and median fins are probably modified 

 scales or lepidotrichia. In the median fins the fin -rays are at first 

 more numerous than their supporting radials, but in the more 

 specialised Teleostomes they ultimately equal them in number. 

 The body is usually invested by an exoskeleton of articulated 

 rhombic or imbricated cycloid scales. Claspers are unknown. 

 In the surviving members of the group there is usually an air- 



' Hence the name "Teleostomi" or "perfect-mouthed" Fishes. 

 475 



