CHAPTER XVIII 
TELEOSTOMI: GENERAL CHARACTERS—CROSSOPTERYGII— 
CHONDROSTEI——HOLOSTEL 
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- 
’ 
1 Hence the name ‘‘ Teleostomi’’ or ‘‘ perfect-mouthed”’ Fishes. 
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