V BONES OF SKULL 343 



the region of the Hyoid arch the Teleostomatous fishes with their 

 greatly developed operculum develop a series of opercular hones. 



Of these various bones mentioned in relation with the buccal 

 cavity and pharynx the majority show more or less distinct evidence of 

 their dental origin [Premaxilla, Maxilla, Dentary, Palatine, Pterygoid, 

 Vomer, Parasphenoid, Opercular — cf. Hertwig, 1874*]. In some cases 

 this may be apparent only in part of the bone, the rest developing 

 as an ordinary investment bone, while in a few cases bone which 

 is in one part of the investing type may in another part present 

 all the features of a replacement bone. 



The student should recognize that the ossification cff the skull, 

 though differing greatly in degree in different Vertebrates, is never 

 complete. In the a.dult Elasmobranch the cranium is entirely 

 cartilaginous, bone being confined to the placoid scales : in a Sturgeon 

 there is still a well-developed chondrocranium but the surface of the 

 head is covered with large bony plates: in such a Teleost as the 

 Salmon the chondrocranium also persists to a great extent but extensive 

 tracts of the cartilage are replaced by bone while the superficial plates 

 of bone are now in much more intimate relations to the surface of the 

 cartilage : in such a Teleost as a Cod again the cartilage is reduced 

 in the adult to such an extent as to be quite inconspicuous. It never 

 however completely disappears and the macerated skull of a Vertebrate 

 as seen in an osteological or palaeontological collection is imperfect, 

 being without parts which may be of great morphological significance. 



Auditory Skeleton. — In those Tetrapoda in which a tympanic 

 membrane is present the vibrations of this membrane are trans- 

 mitted through the tympanic cavity to a movable portion of the 

 wall of the auditory capsule by a special arrangement of skeletal 

 structures. These reach their highest development in the auditory 

 ossicles of the Mammalia which have attracted much attention from 

 students of mammalian anatomy and have been the centre of much 

 controversy as to their phylogenetic origin. In the non-mammalian 

 Vertebrates the two outer members of the chain of ossicles — the 

 malleus and incus — have not yet made their appearance so that we 

 are only concerned in this volume with the inner or stapedial portion 

 which is represented in the Sauropsida and most of the Anura by 

 the columella auris. It will be convenient to study the develop- 

 ment of this in the case of the Lacertilia in which it has been 

 recently investigated by Versluys (1903), Cords (1909) and Good- 

 rich (1915). 



It will be recalled that the tympanic cavity is the dilated outer 

 end of the spiracular or hyomandibular gill pouch, and the Eustachian 

 tube is the inner or pharyngeal portion of this pouch. The pouch is 

 for a time open to the exterior, forming an ordinary spiracular cleft, 

 bounded in front by the mandibular and behind by the hyoid arch. 

 In the hyoid arch is situated the main branch (hyomandibular) of 

 the Pacial nerve and from this, near its dorsal end, there comes off a 

 branch — the chorda tympani — which runs in a ventral direction 



