{221}[Illustration: FIG. 126.—View of the upper surface of the dental plate of _Bdellostoma_. _t_, Tendon of retractor muscle. (From Ayers and Jackson.)] [Illustration: FIG. 127.—Dorsal view of the skull of _Bdellostoma_. Reference letters as in Fig. 125. (After Ayers and Jackson.)] Under this head may be included 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 opposite sides beneath the external portion of the canal, apparently 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-branchial basket to small isolated cartilages in relation with the gill-apertures and the œsophago-cutaneous duct. (vi.) The extraordinary development of the lingual apparatus, of which it has {222}been remarked that it "dominates the whole body, everything else yields to it." Meckel's cartilages are represented either by the cornual cartilages, as seems most probable, or by the dental plate (Fig. 125, _c.c._ and D). [Illustration: FIG. 128.—Lateral view of the skull of _Notidanus_ (_Heptanchus_) _cinereus_; _mck_, Meckel's cartilage, or primitive lower jaw; _pal.qu_, palato-quadrate cartilage or primitive upper jaw; _pt.orb_, post-orbital process of the cranium with which the post-orbital process of the palato-quadrate articulates. (From Parker and Haswell, after Gegenbaur.)]