272 TELEOSTOM1 



tion and passes outwards and backwards over the hyomandibular 

 cartilage, in the Acipenseridae it issues below and passes behind the 

 hyomandibular ; in Amia, Lepidosteus, and the Teleostei it passes 

 through it. Polypterus displays an intermediate condition in which 

 the mandibular branch passes in front and the hyoid branch behind 

 the hyomandibular. Doubtless these differences are due to the 

 shifting forward and upward of the articulation. 



Epihyal (interhyal), ceratohyal, hypohyal, and basihyal elements 

 are generally present (Fig. 331). The basibranchials are usually 

 well represented, and many of the branchial arches may have infra- 

 and suprapharyngeals. As a rule, the branchial arches are 

 extensively ossified. 



There are some interesting points to be noticed concerning the 

 teeth of the Teleostomes which may be mentioned here. As a 

 rule, they develop regularly in a dental fold, succeeding each other 

 throughout life ; but they differ much in size, shape, and mode of 

 attachment (Tornes [440]). While in the Selachii the teeth are 

 merely bound by connective tissue to the jaws, in the Teleostomes 

 they usually become firmly fixed and cemented on to the dermal 

 jaw-bones by bony substance, which is reabsorbed when the tooth 

 is shed. Frequently, however, some of the teeth (Esox, Gadus, 

 Lophius) are movably attached by means of an elastic ligament on 

 the inner side, allowing them to be folded back when food is taken 

 in (Fig. 81). Rarely the teeth are planted in sockets (Sauro- 

 dontidae), to the sides of which they may become anchylosed 

 (Scomber). Sometimes they are placed in a deep groove. The 

 normal process of succession may become much modified. In many 

 fish the new tooth grows underneath the old one so as to replace 

 it 'vertically' (Fig. 448). In others, successive generations of 

 teeth may become cemented together, forming a beak-like covering 

 to the jaws, with cutting edge or grinding surface ; such compound 

 teeth have been independently developed in several families (Dio- 

 dontidae, Figs. 451, 453, p. 440 ; Hoplognathidae, Fig. 439, p. 431). 



Correlated with the presence of a complete opercular flap, we 

 find the septum supporting the branchial lamellae undergoing 

 greater and greater redaction in the Teleostomes. It is short in 

 the Chondrostei, and almost absent in the Teleostei. The lamellae 

 project, therefore, more and more freely into the branchial cavity, 

 and the branchial rays supporting them invariably form a double 

 series on each arch, not a single series as in Selachians (Fig. 57). 



Breathing valves are often developed on the jaws, serving to 

 direct the stream of water through the gills (Dahlgreen [96], 

 Allis [13]). 



The axial and appendicular skeleton become very thoroughly 



