GREGORY: FISH SKULLS 



423 



earlier developmental stages may in many forms tend to be retained in the adult (see p. 431). 

 The Dentition. — The lining membrane of the stomadaeum that covers the primary jaws 

 and the branchial arches not only gives rise to the teeth in the premaxillae, maxilla and 

 dentaries but also to those on the vomers, palatines, ento- and ecto-pterygoids, parasphenoid 

 and coronoid or "splenial"; while from the same source arise the teeth on the pharyngeal 

 surface of the hyoid and branchial arches. 



In predaceous types as a rule the emphasis falls on the teeth of the outer jaws carried 

 by the premaxillae, maxillae and dentaries, as already noted. But in the predaceous pikes, 

 barracudas and other types the inner jaws and roof of the mouth also bristle with a chevaux- 

 de-frise of long, more or less backwardly directed teeth, which obviously function to prevent 

 the escape of the struggling and partly swallowed prey. Similar conditions are found in 

 certain predaceous deep-sea forms such as Omosudis (Fig. 89). In such forms the primary 

 or inner upper jaws (palatopterygoids) will naturally be sufficiently strong to support these 

 large teeth. In the morays (Fig. 825), on the other hand, the palatopterygoid tract de- 

 generates, while the maxilla and vomer bear the principal teeth; the ceratohyals also bear 

 recurved teeth and function like an inner pair of jaws. 



The presence of stout teeth on the palate and inner sides of the jaw in Osteoglossum 

 and Lepidosteus necessitates extra bracing of the palate (see pp. 164, 129). 



Crushing teeth are developed on the massive premaxillae and dentaries of the sparids 

 (Fig. 123) and in Anarrhichas (Fig. 255) and bracing bones of the jaws are very strongly 

 built. Figure 291 illustrates five of the principle types of jaws, namely, the normal {A), 

 the predaceous (/)), the small-mouthed or nibbling {B), the crushing (C) and the needle-gar 

 {E) types. These have been evolved more than once independently among the lower and 

 the higher fishes, as in the following examples: 



I 



Norma! 



(with small 



teeth) 



II 



Predaceous 



(usually with 



large sharp teeth) 



Small-mouthed, 

 nibbling 



IV 



Crushing 



(with molars 



or tritors) 



V 



Needle- gar 



(with or 



without teeth) 



Crossopterygi 

 Palaeonlscoids 



Holostei 



Isospondyli. . 



Ostariophysi . 

 Percomorphi . 



OsUolepis 

 TrissoUpis 



Amia 

 Salvinellus 



Erythrinus 

 Micropterus 



Migalichthys 

 Cheirolepis 



Protosphryana 

 Astronesthes 



Hydrocyon 

 Trichiurus 



Mesolepis 



Mesodon 

 Chatoessus 

 (no teeth) 

 Disiichodus 

 Chatodon 



Mylacanthus 

 Cheirodus 

 (tritors) 

 Lepidotus 



Archosargus 



Saurichthys 

 Aspidorhynchus 



Tylosvrus 



In addition to these types the edentulous and protrusile types are noticed below. 



In the extinct pycnodonts the teeth on the roof of the jaw became hemispherical and 

 were arranged on a tapering cylindrical surface. They were opposed by similar teeth ar- 

 ranged in a U-shaped trough supported by the inner sides and floor of the mandible. In the 

 Cretaceous and later Elopidae, Albulidae and related families, the roof of the mouth is 

 thickly strewn with granular or small, more or less circular teeth, which oppose similar 

 teeth on the tongue (glossohyal) and adjacent parts (Woodward, 1901, p. vii; Ridewood, 

 1904a, p. 50). In Gonorhynchus a circular patch of blunt teeth on the second basibranchial 

 engages with teeth on the entopterygoids. 



Pharyngeal teeth, on the fourth pharyngo-branchials in the upper, and on the fifth 



