VISCERAL ARCHES 271 



and endopterygoid (mesopterygoid) ; bones which often are toothed. 

 An endochondral quadrate bears the lower jaw ; and an endo- 

 chondral metapterygoid joins on to the hyomandibular above it. 

 Thus the hyomandibular, symplectic (when present), and palato- 

 ptery go-quadrate usually form one rigid arch. How far these 

 three pterygoid bones are all present in primitive and early forms 

 is uncertain. 



The palato-pterygoid arch articulates in front with the ethmoid 

 cartilage laterally, by an ethmo- palatine articulation. The right 

 and left arches do not, therefore, meet below the snout as in 

 the Chondrichthyes (except in the Acipenseroidei). It is interest- 

 ing to notice that in certain (perhaps in all the primitive forms) of 

 the lower Teleostomes (Pycnodontidae, Aspidorhynchidae, Macro- 

 semiidae, Pholidophoridae, Lepidosteidae, Lepidotidae, Amiidae) the 

 metapterygoid sends inwards a process (Fig 239), which quite or 



Inner view of the lower jaw of Amia calva, L. (After Allis.) a, articular ; an, angular ; 

 ar, dermarticular ; a.sp, anterior splenials ; cp, coronoid cartilage ; d, dentary ; so,, supra- 

 angular ; sm, mento-Meckelian ; sp, splenial with minute teeth ; t, marginal tooth. 



nearly articulates with the trabecular region of the skull between 

 the optic and the trigeminal foramina (van Wijhe [494], Reis [349]) ; 

 a vestige of this process appears in the embryos of some- Teleostei 

 (Salmo, Swinnerton [431], Winslow). This connection may 

 perhaps represent the lower articulation of the quadrate in auto- 

 stylic skulls the pedicle of the suspensorium (p. 97). In that 

 case it would point to the Teleostomi having preserved a trace 

 of the original connection between the mandibular arch and the 

 cranium (p. 97). 



The large cartilaginous hyomandibular generally ossifies in two 

 pieces a hyomandibular bone above articulating with the skull, 

 and a symplectic below, connected with the quadrate (Fig. 239). 

 In Polypterus, however, there is no symplectic (see p. 295) ; and 

 in some Teleosts, such as the Siluridae and Anguillifermes (Apodes), 

 it appears to have been lost. 



The different relation of the seventh nerve to the hyomandibular 

 is of some interest (van Wijhe). Whereas in Selachians the main 

 hyomandibular branch issues from the skull in front of the articula- 



