220 FISHES CHAP. 
extra-branchial cartilages of an Elasmobranch than with the 
more deeply-seated branchial arches. 
While the skull of the Myxinoid Cyclostomes?* is constructed 
on the same general lines as that of the Lamprey, it is in some 
respects more primitive. It is also clear that in other features 
the skull has undergone marked specialisation on lines of its 
own, and in some points again it seems to deviate less from the 
more normal Craniate type. Of the more obvious differences, as 
illustrated by the skull of Bdellostoma (Figs. 125-127), it will 
Fic. 125.—Side view of the skull of Bdellostoma ; the gill-apertures and their cartilages 
have been omitted. A, Auditory capsule; B, B’, B’, the anterior, middle, and 
posterior segments of the lingual bar; 01, cartilage connecting the hyoid arch with 
the second branchial arch ; br, br?, first and second branchial arches ; v.c, coronal 
cartilage ; Or, cranium; D, dental plate ; dt, median dorsal tooth; Hz.n.c, ex- 
ternal part of the naso-pituitary canal; Hp, hypophysial plate; Hy, hyoid arch ; 
NV. subnasal cartilage; mc, neural canal; Nf, notochord; OC, olfactory capsule ; 
PL, palatine portion of the palato-quadrate cartilage PQ, S, supra-pharyngeal 
plate supporting the velum ; ¢, tendon of the retractor mandibuli muscle ; ¢1, 2, ‘he 
tentacular cartilages ; {, cartilage supporting mouth lobe; ¢7, trabecula; V1, rod 
connecting S with the inner surface of the hyoid arch of its side; V, outer lateral 
rod which joins V!; 1, 2, 3, fenestrae. (Modified from Ayers and Jackson.) 
be sufficient here to mention the following: (i.) The more primitive 
condition of the chondrocranium, the roof and side walls of the 
cranial cavity being entirely membranous. (ii.) The non-develop- 
ment of a suctorial buccal funnel and the presence of oral tentacles, 
associated with the absence of the complex system of labial 
cartilages and the substitution of a special tentacular skeleton. 
(iii.) The special modifications induced by the length and physio- 
logical importance of the naso-pituitary canal and by its com- 
munication with the pharynx after perforating the pituitary 
fontanelle in the cranial floor. Under this head may be included 
1 W. K. Parker, Phil. Trans. 174, 1888, pp. 376-405 ; Ayers and Jackson, Journ. 
Morph. xvii. 1901, p. 193. 
