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Show 19G ON THE STRUCTURE OF THE SJOJLL. in that it lies above, and not below, the forward continuation of the cranial cavity (Fig. 78). Usually, an inter-orbital septum is fonned by the compressed floor of the skull; here it is constituted by the compressed roof. Two c~ambers f?r t~e olfactory sacs (N, Na) terminate the skull antenorly and Infenorly; and they, the lips and the anterior part of the snout, are protected and supported by a number of accessory cartilage (d tom). Below the auditory and orbital regions, and in front of the latter as far as the nasal capsules, the base of the skull gives off a broad cartilaginous sub-ocular plate (0, D, Fio·. 78), the two I~ ig. 78. Fig. 78.-Vertical section of the kull of Ghimce1·a monstrosrt without the labial and nasal cartilages.-A, the basi-occipital r gion; P, the pituitary fossa; Na, the partition between the two olfactory sacs· B, the alveolu for the anterior upper jaw tooth; I. Or., the inter-orbital septum ; 'asc, psc, anterior and posterior vertical semicircular canals; I., II., V., Vll I., exits of the olfactory, optic, fifth, aud eighth pairs of cerebral nerves. edges of which, sloping towards one another, bring it to a point at n (Fig. 77; D, Fjg. 78). With this part of the plate the mandible (Mn) is articulated, while to the middle of its posterior margin (D, 0, Fig. 78) the hyoidean apparatus ( o, Fig. 77) is attached. A vertical section of the skull (Fig. 78) shows that the proper cranial cavity consists of a large posterior chamber, divided by a long and comparatively narrow neck from a much smaller, but still large, anterior chamber. The latter contains the olfactory lobes, and presents on each side in front a sort of cribriform plate, through which the filamen' ts of the' olfactory nerve pass 'l'HE SKULLS OF FISHES. 197 to the nasal sacs. The commencement of the narrow neek iB perforated on each side by the optic foramina (II.). The hinder dilatation contctins the mass of the brain, and, on each side, chu.1nbers for the auditory organs, which communicate with it, are situated. The posterior edge of the inter-orbital septum bounds this chamber in front, above the " neck." In front of the anterior boundary of the inter-orbital septum, and above the olfactory division of the skull cavity, there is a curious chamber filled with fatty matter, and open in front and behind, which is traversed by the nasal division of the fifth nerve. Muller well says, "The skull of Ohimmra is most like that of a tadpole ; "* but if we interpret the former strictly by the latter, as I believe ought to be done, the results will be somewhat different from those at which Muller arrives. The plate 0, D answers precisely to the sub-ocular arch of the lamprey and to the corresponding arch in the tadpole's skull, though it is chondrified throughout, and not perforated by a large aperture, as in the two ]atter animals. But, admitting this, the further development of the frog proves that the sub-ocular arch answers to the common suspensorium of the hyoid and mandible, and to the palatine, pterygoid, and quadrate bones; and that it has nothing to do with the maxilla or premaxilla. The large posterior upper jaw teeth of the Chim.ooroids (a), therefore, being attached to the under surface of the anterior part of the subocular plate, must be palatine or palato-pterygoid teeth. The small anterior teeth (a), on the other hand, are fitted into fossm, or alveoli (B), which are situated immediately under the floor of the nasal chambers, in the vomerine region of the skull, and must be regarded as vomerine teeth-as, indeed, Cuvier suo·- o gested. On the other hand, I think Muller's view that the cartilages ( d, e, &c.) are accessory labial cartilages, and not, as Cuvier supposed, representatives of premaxilhe, maxillm, &c., has everything in its favour. c. The cartilaginous cranium, with a mandible and with a moveable suspensorium. This fonn. of cranium is met with in the sharks and rays, and may be Illustrated by an account of that of Squatina, the * Vorgloiehoudc Auutomie dor Myxinuiden. Erstcr 'l'b.oil, p. 150. |