236 BONY FISHES vm. 4- 



gar-pikes come to the surface to gulp air. On the other hand, it has 

 developed certain special features, especially the long jaws, with which 

 it catches other fishes, and the nearly symmetrical tail. Fossils similar 

 to the modern gar-pike are found in the Eocene. 



Some of the later holosteans became deep- and short-bodied and 

 developed a small mouth with flat crushing teeth or a beak, for 

 instance, *Lepidotes (Trias to Cretaceous), and *Dapedius (Jurassic). 

 They probably browsed on corals, like the modern parrot fishes 

 (Scaridae). *Microdon and other 'pycnodonts' became laterally 

 flattened, like some palaeoniscids and the modern sea butterflies 

 (Chaetodontidae). 



Another line of holostean evolution, developing from the original 

 stock, retained the streamlined body and from these both the modern 

 teleosteans and the amioids were evolved (Fig. 147). *Caturus (Trias 

 to Cretaceous) was covered with thick scales, but in *Pachycormns 

 (Cretaceous) they are thinner; these were active pelagic predators. 

 In Amia the scales became reduced to single bony cycloid scales, as in 

 Teleostei. Meanwhile other changes took place, the tail. fin becoming 

 externally completely symmetrical and the maxilla and other cheek 

 bones reduced. Amia has nearly reached the teleostean stage but 

 retains certain primitive features in the skeleton and the small eggs 

 with holoblastic cleavage. 



5. Superorder 3. Teleostei 



The groups so far considered have been nearly completely replaced 

 by the Teleostei, fishes derived from a holostean stock, which have 

 carried still farther the tendencies to shortening and symmetry of the 

 tail, reduction of the scales, and various changes in the skull, such as 

 reduction of the maxilla. The type apparently arose in the sea in late 

 Triassic times, but remained rare until the Cretaceous, by which time 

 several different lines of evolution had already begun *Pholidophorus 

 from the Trias still carried an armour of thick scales but may well 

 have given rise to *Leptolepis from the Jurassic and Cretaceous, which 

 is generally considered to be close to the ancestry of all Teleostei and 

 may be placed close to the order Isospondyli, many of which are 

 still alive. *Leptolepis was a long-bodied fish with the pelvic fins placed 

 far back, a skull with a full complement of bones, and a large maxilla. 

 The scales still show traces of the ganoin layer. 



From some fish like these leptolepids have been derived the 20,000 

 or more species of bony fish found today. It is natural that in any 

 group of animals that has evolved relatively recently classification will 



