THE TELEOSTEI. 139 



4. Beak-like compound teeth, attached to the pi-emaxillaa 

 «!id dentary bones of the mandible. 



These are of two kinds. In the Parrotfisli {Scm'us) the 

 beak is formed by the union of numerous separately-developed 

 teeth into one mass. But in the Gymnodonts (Tetrodon and 

 Dio'Jon) the beak is produced by the coalescence of broad 

 calcified horizontal lamellije thrown off from a subjacent pulp. 



5. In the Carp and its allies the basi-occipital sends down 

 a median process, which expands at the end, and supports a 

 broad, thick, horny tooth. 



The stomach is usually wide and sac-like, but sometimes 

 (in Scomberesoces, Gyprlnoids, and others) is not wider than 

 the intestine. Occasionally, as in Mugil, it acquires thick 

 walls and becomes gizzard-like. The commencement of the 

 small intestine is very generally marked bj' the presence of 

 more or less numerous caecal diverticula, the pyloric cceca. 

 The small intestine has no spiral valve, though the mucous 

 membrane may be raised into large transverse folds. The rec- 

 tum does not terminate in a cloaca, and almost always opens 

 quite separately from the urinary and genital ducts, and in 

 front of them. 



In many Teleostean fishes an air-Madder underlies the ver- 

 tebral column, and is connected by an open pneumatic duct 

 vrith the dorsal wall of the oesophagus, or even with the stom- 

 ach, as in the Herring. In other Teleostei, the air-bladder oc- 

 cupies the same position, but is closed, the duct by which the 

 air-bladder is primitively connected with the alimentary canal 

 becoming obliterated. In a comparatively small number of 

 the Teleostei — the Slennii, the Pleuronectidce or Flatfishes, 

 the Sand-eel (Ammodytes), the Zioricarini, and Symhranchii, 

 and some members of other families — there is no air-bladder. 

 In those Teleostei in -which it is present, it may be divided into 

 two parts by a constriction ; or it may be prolonged into di- 

 verticula ; or retia mirabilia may be developed in its walls. 

 Sometimes the air-bladder is brought into direct relation with 

 the membranous labyrinth, as in Myripristis and Sparus, and 

 the Herring, Shad, and Anchovy — ^prolongations of the one or- 

 gan being separated from the other only by a membranous 

 fenestra in the wall of the skull. In the Sihiroidel, Gypri- 

 noidei, and Charaeini, and in the Gymnotini, the anterior 

 end of the air-bladder is connected with the membranous vesti- 

 bule by the intermediation of a series of bones attached to the 

 vertebral column, some of which are movable. 



The vessels of the air-bladder are derived from, and empty 



