VIII SleUlels 221 
the depression of the mesethmoid or hypophysial plate for the 
support of the naso-pituitary canal, the forward prolongation and 
median union of the palato- 
quadrate cartilages of oppo- 
site sides beneath the external 
portion of the canal, appar- 
ently for the support of the 
latter,and the encircling of the 
canal by supporting annular 
rings of cartilage. (iv.) The 
presence of two branchial 
arches, connected, as in 
Fishes, with a median basi- 
branchial segment which 
forms the middle one of the 
three divisions of the lingual 
apparatus. (v.) The reduction 
of the complicated extra- 
Fig. 126.—View of the upper surface Fic, 127.—Dorsal view of the skull of Bdello- 
of the dental plate of Bdellostoma. stoma. Reference letters as in Fig. 125. 
t, Tendon of retractor muscle. (From (After Ayers and Jackson. ) 
Ayers and Jackson.) 
branchial basket to small isolated cartilages in relation with the 
gill-apertures and the cesophago-cutaneous duct. (vi.) The extra- 
ordinary development of the lingual apparatus, of which it has 
