. Elements of Comparative Anatomy. 450 COMPAEATIVE Al^ATOMY. only part of tlie first arcli which, is movable. The partly ossified skull of the Dipnoi has just the same characters. §342. The cartilaginous cranium is most completely retained in the Sturiones (Fig. 241) ; in the rest of the Ganoidei it is partly so ; in the Teleostei, and especially in Salmo and Esox, it is more or less so. In most forms the ethmoidal portion remains cartilaginous. Henceforward^ and all through the Vertebrate phylum, a cartilaginous cranium can be made out in the earliest rudiments of the skull; howsoever modifie
. Elements of Comparative Anatomy. 450 COMPAEATIVE Al^ATOMY. only part of tlie first arcli which, is movable. The partly ossified skull of the Dipnoi has just the same characters. §342. The cartilaginous cranium is most completely retained in the Sturiones (Fig. 241) ; in the rest of the Ganoidei it is partly so ; in the Teleostei, and especially in Salmo and Esox, it is more or less so. In most forms the ethmoidal portion remains cartilaginous. Henceforward^ and all through the Vertebrate phylum, a cartilaginous cranium can be made out in the earliest rudiments of the skull; howsoever modified, it may be derived from the primitive condition, and may therefore be regarded as a remnant of that condition. The degenerations which affect this primordial cranium are chiefly due to the ossifications which take place in it. Bony parts, which perform the function of organs of support and protection, better than cartilage does, take its place; and the development of these bones,. Fig. 241. Cephalic skeleton of Acipenser sturio; after the removal of the cover- ing bones, r Eostrum. n Nasal cavity, o Optic foramen, tr Trigeminal foramen. s}} Spinous processes of the anterior portion of the vertebral column, whicli is fuserl with the cranium, p Palato-quadrate. m Mandible. Hm. Hyomandibular. s Sym. plectic. fcr Branchial arches, r Eibs. which enter into connection with the cartilaginous cranium, is clearly the cause of the degeneration of the cartilaginous tissue. A higher and more complete arrangement has ousted the lower one. Bony pieces become connected with the cartilaginous pieces of the branchial skeleton, as well as with the cartilaginous cranium; in this way the whole cephalic skeleton gradually passes from a cartilaginous into an osseous condition. The osseous elements which go to make it up are almost all derived from the dermal structures, which we met with as dermal denticles in the Selachii. Some of these bones appear on the outer surface of the carti- laginous cr
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