220 



FISHES 



extra -branchial cartilages of an Elasmobranch than with the 

 more deeply-seated branchial arches. 



While the skull of the Myxinoid Cyclostomes l 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 



FIG. 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 ; b\ cartilage connecting the hyoid arch with 

 the second branchial arch ; br l , br"*, first and second branchial arches ; c.c, coronal 

 cartilage ; Or, cranium ; D, dental plate ; dt, median dorsal tooth ; Ex.n.c, ex- 

 ternal part of the naso- pituitary canal ; ffp, hypophysial plate ; Hy, hyoid arch ; 

 JV. subnasal cartilage ; nc, neural canal ; Nt, notochord ; OC, olfactory capsule ; 

 PL, palatine portion of the palato - quadrate cartilage PQ ; S, supra -pharyngeal 

 plate supporting the velum ; t, tendon of the retractor mandibnli muscle ; t l , (~, t 3 , 

 tentacular cartilages ; t 1 , cartilage supporting mouth lobe ; tr, trabecula ; T 71 , rod 

 connecting S with the inner surface of the hyoid arch of its side ; V, outer lateral 

 rod which joins V 1 ; 1, 2, 3, fenestrae. (Modified from Ayers and Jackson.) 



be sufficient here to mention the following : (i.) The more primitive 

 condition of the choiidrocranium, 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, 1883, pp. 376-405 ; Ayers and Jackson, Journ. 

 Morph. xvii. 1901, p. 193. 



