PISCES. TELEOSTEI. 69 



Order 4. TELEOSTEI. 



The exoskeleton is sometimes absent but generally consists 

 of overlapping cycloid or ctenoid scales. Bony plates are some- 

 times present, as in the Siluridae, or the body may be encased 

 in a complete armour of calcined plates, as in Ostracion. 

 Enamel is however never present, and the plates are entirely 

 mesodermal. The skeleton is bony, but in the skull much 

 cartilage generally remains. The vertebral centra are usually 

 deeply biconcave, and the tail is of the masked heterocercal 

 type distinguished as homocercal. In the skull the occipital 

 region is always completely ossified, while the sphenoidal 

 region is generally less ossified. The skull has usually 

 a very large number of membrane bones developed in connec- 

 tion with it. The teeth vary much in character in the different 

 members of the order, but are as a rule numerous and pointed, 

 and are ankylosed to the bone. The suspensorium is hyo- 

 stylic and the jaws have much the same arrangement as in 

 the Holostei. There are five pairs of branchial arches, of 

 which all except the last bear gill rays. A series of dermal 

 opercular bones is developed in connection with these arches. 

 The pectoral girdle consists almost entirely of dermal clavicular 

 bones. The pelvic girdle has disappeared, its place being taken 

 by the enlarged and ossified dermal fin-rays of the pelvic fins. 



The group includes the vast majority of living fish (see 

 p. 33). 



Order 5. DIPNOI. 



The exoskeleton is of two types ; dermal bones are largely 

 developed in the head region, while the tail and posterior 

 part of the body may be naked or may be covered with over- 

 lapping scales. The cranium remains chiefly cartilaginous, 

 the palato-pterygo-quadrate bar is fused with the cranium, 

 and the suspensorium is autostylic. The gill clefts^ are feebly 

 developed and open into a cavity covered by an operculum. 

 The notochord is persistent and unconstricted, and the limbs 

 are archipterygia. The pelvic fins are without claspers. 



