viii. 6 TELEOSTS 237 



be difficult, because the separate twigs of the evolutionary bush will 

 show little difference from each other and there may be much parallel 

 evolution. It is only when intermediate forms have become extinct 

 that clear-cut major groups appear. It is therefore not easy to find 

 useful subdivisions of Teleostei; we may divide them among five 

 orders but most classifications require many more. 



The first, Isospondyli, fish with soft rays, show primitive features 

 in the large maxilla, which forms the posterior margin of the upper 

 jaw, the persistence of an open duct to the air-bladder, and the 

 posterior position of the pelvic fins. Fishes of essentially similar type 

 are known as far back as the Cretaceous (* Port hens). Several familiar 

 fishes are of this type, including the salmon and trout (Salmo) and 

 herrings (Clupea). The order Ostariophysi is a large group of fresh- 

 water fishes, related to the Isospondyli. The anterior vertebrae are 

 modified to form a chain of bones, the Weberian ossicles, joining the 

 swim-bladder to the ear (p. 217). Here belong the carp and gold- 

 fish (Cyprinus), roach (Leucisciis), and cat-fishes (Silurus). The eels 

 (order Apodes) are rather isolated teleosts that diverged early from 

 the main stock and retain many primitive features. The fourth order, 

 the Mesichthyes, includes fishes such as the pike (Esox) and stickle- 

 back (G aster osteus) of structure intermediate between that of the 

 more primitive forms and the latest spiny-finned teleosts. The 

 pipe-fishes {Syngnathus) and sea-horses (Hippocampus) are probably 

 related to the sticklebacks. The flying-fish (Exocoetus) also belongs in 

 this group. 



The members of the order Acanthopterygii are the most highly 

 developed fishes, characterized by the stiff spines at the front of the 

 dorsal and anal fins. The maxilla is short, the duct of the air-bladder 

 is closed, the body shortened, and the pelvic fins far forward. Fishes 

 of this type already existed in the Cretaceous (*Hoplopteryx) and the 

 condition may have been evolved along several different lines; the 

 group includes a vast array of modern types. Here belong the perches 

 (Perca), mullet (Mugil), wrasse (Labrus), John Dories (Zeus), blennies 

 (Blennius) as well as the gadid fishes such as the whiting (Gadus). 

 Anglers (Lophius), gurnards (Trigla), and the flat-fishes, the plaice 

 (Pleuronectes) and sole (Solea) and others, are further members of 

 this very large order. 



6. Analysis of evolution of the Actinopterygii 



Our knowledge of the history of the Actinopterygii is sufficiently 

 complete for us to be able to state more definite conclusions about the 



