In Reptiles, the prootic and opisthotic frequently remain distinct even in the adult.
In Birds, the epiotic and opisthotic are early united with the supra- and exoccipital; and at a later period the prootic is also indistinguishably fused with the adjacent parts.
In Mammals the three ossifications fuse into a continuous whole—the periotic bone—which may be partially united with the adjacent parts.
In the pituitary region of the base of the cranium a pair of osseous centres or in the higher types a single centre (Parker[209]) gives rise to the basisphenoid bone, and in front of this another basal or pair of basal ossifications forms the presphenoid, while laterally to these two centres there are formed centres of ossification in the alisphenoid and orbitosphenoid regions, which may be extremely reduced in various Sauropsida, leaving the side walls of the skull almost entirely formed of membrane or cartilage.
In the ethmoid region there may arise a median ossification forming the mesethmoid and lateral ossifications forming the lateral ethmoids or prefrontals; which may assist in forming the front wall of the brain-case, or be situated quite externally to the brain-case and be only related to the olfactory capsules.
The labial cartilages. In most Fishes a series of skeletal structures, known as the labial cartilages, are developed at the front and sides of the mouth, and in connection with the olfactory capsules; and these cartilages still persist in connection with the olfactory capsules, though in a reduced form, in the higher types. They are more developed in the Cyclostomata than in any other Vertebrate type.
The meaning of these cartilages is very obscure; but, from their being in part employed to support the lips and horny teeth of the Cyclostomata and the Tadpole, I should be inclined to regard them as remnants of a primitive skeleton supporting the suctorial mouth, with which, on the grounds already stated (p. [317]), I believe the ancestors of the present Vertebrata to have been provided.
(439) A. Dugès. “Recherches sur l'Ostéologie et la myologie des Batraciens à leur différents âges.” Paris, Mém. savans étrang. 1835, and An. Sci. Nat. Vol. I. 1834.
(440) C. Gegenbaur. Untersuchungen z. vergleich. Anat. d. Wirbelthiere, III. Heft. Das Kopfskelet d. Selachier. Leipzig, 1872.
(441) Günther. Beob. üb. die Entwick. d. Gehörorgans. Leipzig, 1842.
(442) O. Hertwig. “Ueb. d. Zahnsystem d. Amphibien u. seine Bedeutung f. d. Genese d. Skelets d. Mundhöhle.” Archiv f. mikr. Anat., Vol. XI. 1874, suppl.
(443) T. H. Huxley. “On the theory of the vertebrate skull.” Proc. Royal Soc., Vol. ix. 1858.
(444) T. H. Huxley. The Elements of Comparative Anatomy. London, 1869.
(445) T. H. Huxley. “On the Malleus and Incus.” Proc. Zool. Soc., 1869.
(446) T. H. Huxley. “On Ceratodus Forsteri.” Proc. Zool. Soc., 1876.
(447) T. H. Huxley. “The nature of the craniofacial apparatus of Petromyzon.” Journ. of Anat. and Phys., Vol. X. 1876.
(448) T. H. Huxley. The Anatomy of Vertebrated Animals. London, 1871.
(449) W. K. Parker. “On the structure and development of the skull of the Common Fowl (Gallus Domesticus).” Phil. Trans., 1869.
(450) W. K. Parker. “On the structure and development of the skull of the Common Frog (Rana temporaria).” Phil. Trans., 1871.
(451) W. K. Parker. “On the structure and development of the skull in the Salmon (Salmo salar).” Bakerian Lecture, Phil. Trans., 1873.
(452) W. K. Parker. “On the structure and development of the skull in the Pig (Sus scrofa).” Phil. Trans., 1874.
(453) W. K. Parker. “On the structure and development of the skull in the Batrachia.” Part II. Phil. Trans., 1876.
(454) W. K. Parker. “On the structure and development of the skull in the Urodelous Amphibia.” Part III. Phil. Trans., 1877.
(455) W. K. Parker. “On the structure and development of the skull in the Common Snake (Tropidonotus natrix).” Phil. Trans., 1878.
(456) W. K. Parker. “On the structure and development of the skull in Sharks and Skates.” Trans. Zoolog. Soc., 1878. Vol. X. pt. IV.
(457) W. K. Parker. “On the structure and development of the skull in the Lacertilia.” Pt. I. Lacerta agilis, L. viridis and Zootoca vivipara. Phil. Trans., 1879.
(458) W. K. Parker. “The development of the Green Turtle.” The Zoology of the Voyage of H. M. S. Challenger. Vol. I. pt. V.
(459) W. K. Parker. “The structure and development of the skull in the Batrachia.” Pt. III. Phil. Trans., 1880.
(460) W. K. Parker and G. T. Bettany. The Morphology of the Skull. London, 1877.
(460*) H. Rathke. Entwick. d. Natter. Königsberg, 1839.
(461) C. B. Reichert. “Ueber die Visceralbogen d. Wirbelthiere.” Müller’s Archiv, 1837.
(462) W. Salensky. “Beiträge z. Entwick. d. knorpeligen Gehörknöchelchen.” Morphol. Jahrbuch, Vol. VI. 1880.
Vide also Kölliker (No. [298]), especially for the human and mammalian skull; Götte (No. [296]).