294 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



hypopharyngeal being attached to the 5th arch, while the epiphaiyn- 

 geal is in relation to the 3rd and 4th arches. A stronger proof of 

 this fact, however, is afforded by a study of the development of these 

 bones. They are then seen to be originally quite distinct from the 

 adjacent cartilaginous branchial arches, and to be formed by the 

 union of the cement-plates of the teeth which they bear, and by a 

 subsequent formation of osseous trabecule by osteoblasts. Their 

 morphological significance is not hard to determine. They represent 

 the remains of the dermal denticles which originally lined the mucous 

 membrane of the buccal and branchial cavities, and which are still 

 to be seen in those situations in certain Selaclii 1 . 



V.— THE SPINAL COLUMN. 

 With regard to this portion of the skeleton, the greatest interest 

 centres round the first four vertebra? and their arches, which have 

 become very much modified in accordance with the development of 

 a series of ossicles within the auditory apparatus and the air-bladder. 

 These anterior vertebra? being thus intimately connected with the 

 auditory sense-organ, will, with greater appropriateness be described 

 in detail in the portion of this work, by Professor Wright, referring 

 to that structure. It will be necessary, however, to denote here 

 briefly the modifications undergone. The body of the first vertebra 

 is fully formed, but its transverse processes are rudimentary, while 

 its dorsal arch forms the stapes of either side, and a pair of inter- 

 crural cartilages present in front of it, are converted into the claus- 

 tra. The body of the second vertebra has entirely disappeared, and 

 become fused with the third, the fusion being indicated by two 

 nutritive foramina at the base of the conjoined vertebrae. Its trans- 

 verse process is wanting, and its dorsal arch becomes converted into 

 the rudimentary incus. The body of the third fuses with the second 

 and fourth ; its dorsal arch is normal, and its spine is represented by 

 the anteriorly directed process, which, arising from the broad flat 

 plate mentioned below, extends forwards and articulates with the 

 supraoccipital and exoccipitals ; and its transverse process is trans- 

 formed into the malleus. The fourth vertebra is fused with the third 

 and fifth ; its transverse process is the broad plate extending out on 

 either side in this region, and its dorsal arch is the backwardly pro- 



1 0. Hertivig Ueber das Zahusystem der Amphibien. Arch, fur mikr. Anat. Bd. XI. supple- 

 ment 1874. See also Jenaische Zeitsch. Bd. VIII. 1874. 



