OCR Text |
Show 19-1 ON THE STRUOTURE Ol!' THE SKULL. olfactory capsule. The eye lies ov r the triangular spaco inclosed between the sides of the skull and these two processes, so that e, g, j may be tern1ed the sub-oc~dar arch. If the skull is viewed from below, the processes e and e of opposite sides are se n to be continued into one another by n, transverse band of cartilage, which forms the proper anterior boundary of the skull. Tho front edge of this band, which 1\fiiller calls the "hard palate," articulates with the broad and expanded cartilaginous plate (a). The common roots of the processes f and h are also continued into a " basi-occipital" plate of cartilage, but, between this plate and the "hard palate," there is an oval space through which the neck of the long olfactory coocum (o, Fig. 75) pas es. This cmcum, therefore, separates the front part of the floor of the cranial cavity, which is simply membranous, from the so-called "hard palate." On comparing this skull with that of the en1bryonic fi sh (see Fig. 72), h obviously answers to the stylo-hyal cartilage ; j, to the ascending posterioi: crus of the palato-quadrate inverted arch and the hyomandibular cartilage ; e, to the ascen<ling and anterior crus of the same. It is true that no natural division of the arch into palato-quadrate and hyomandibular (and symplectic) portions occuts jn the lamprey, bnt this is only one of several respects in which the 1\Iarsipobranehs resmnble Amphibia rather than osseous fishes. The inverted cartilaginous arch which gives attachment to the hyoidean and mandibular apparatuses of a tadpole is strietly comparable to the arch (e, g, f) in the lamprey. The margins of the oval space upon the base of the skull answer to the divergent trabeculm cranii, and the plate a to the ethmovomerine cartilage. ~rhe remarkable and apparently anomalous separation of the basis cranii into an upper membranous and a lower cartilaginous part, by the interposition of the bacl\ ward prolongation of the olfactory chamber, seems to me to be comparable to that separation of the upper and lower walls of the pre-sphenoid, basi-sphenoid, and even of the basi-occipital, by a backward extension of the olfactory cavities, which takes place in so many of the Mammalia. On the other hand, I doubt whether the accessory buccal cartilages, 1, 2, 3, &c., can be strictly compared with anything in other fishes, though some of 'l'IIE SKULLS OF FISHES. 195 them are doubtless, as Muller has suggested, the analogues of labial cartilages. b. The cartilaginous craniurn with a mandible and a fixed suspensoriurn. The Holocephali, or Chimreroid fishes ( Ohimmra and Oallorhynchus) present this type of cranial organization. In accordance with the large development of the brain, the skull of these fishes has attained a great ad vance in dimensions over the spinal Fig. 77. Fig. 77.-:S~ull of Callor?tynchus .Antarcticus (after Muller).-a, anterior tooth of the up~e1 Jaw ; c, postenor tooth ; b, mandibular tooth · d e f, g h z' k z 1 • labtal nasal d . t l '] ' ' ' ' ' ' , ' , n, accesso1y ~u • o:ts the ,han. ros ra ca.rtt ages ; .n, quadrate portion of the sub-ocular plate which . PP. ~otd (?) an.d the mandible.(Mn); p, the representatives of brancrnoste al 1ays, q, the b1anclual a1ches; A~the fifth nerve. t, audttory l'CO'ion · Or orb1't. Vl . 1 d' · · g f e. • ., , , nasa tvlston o c.o lumn, and. pre. sents a large internal chamber· It I·s a con-ti~ uous .cartilaginous mass, without any superior aperture of suf-ficn~ nt size to deserve the name of a fontanelle, in the base of wh~ch the notochord does not persist, and which is definitely articulated by t':o lateral convex fttcets and a median concave surfa~e ou the hinder margin of its floor (A, Fig. 78) with the antenor segment of the spinal column·. t .·The ~kull i~ ~1igh and compressed from side to side; pos-enorly, It exhi~Its, on each side, an enlargement (Au), which lod?es the auditory organ. In front of these are the large orbits (Or.): se~arat~d by a ~hin membranous inter-orbital soptun1 (LOr.), winch Is unhko the Inter-orbital septum usually met with, 0 2 |