VERTEBRATA. 



339 



at the most five bones, pro-, sphen-, pter-, ep-, and opisth-, otics, but the 

 second and third of the five are frequently absent. A basi- and two 

 ali-, sphenoids appear in front of the ear-capsule, and in front of them in 

 turn a prae- and two orbito-, sphenoids. The nasal septum ossifies as a 

 mesethmoid and the lateral masses each as an ecto-ethmoid bone. A 

 palatine, pterygoid, and sometimes a meso-pterygoid appear in the palato- 

 pterygoid bar. The quadrate region forms the quadrate bone, and in 

 Teleostei and bony Ganoidei a meta-pterygoid also. The mandible has 

 generally an articular ossification at its proximal end and occasionally 

 an angular. An ossification sometimes found at the distal end of each 

 mandible, and known as Mento-meckelian, is a persistent lower labial 

 cartilage. The hyomandibular element of the hyoid arch has, in Teleostei 

 and bony Ganoidei, two ossifications the hyomandibular and the sym- 

 plectic. It forms the simple stapes of some Amphibia and Reptilia and 

 the Mammalia ; the complex chain of stapedial elements (medio-, extra-, 

 infra-stapedials) of Anuran Amphibia, the Crocodile, and Aves. The other 

 segments of the hyoid ossify separately, but are sometimes more or less 

 represented by ligament. The segments of the branchial arches may be 

 similarly ossified. In adult Amniota the first only is represented, and is 

 large in some Reptilia and in Aves ; the rest abort. 



To this cartilaginous skull, as above-described, are added a series of 

 bones, developed in the first instance from the skin or mucous membrane 

 of the mouth, but engrafting themselves in higher forms upon the skull 

 or the cranium proper. The principal bones in this series, often spoken 

 of as ' membrane ' bones, are paired parietals, frontals, nasals, lacrymals, 

 upon the dorsal aspect of the cranium ; a vomer single or double, and 

 except in Mammalia, where it is scarcely identifiable, a parasphenoid, on 

 its oral aspect. To the palato-pterygoid bar are added praemaxillae, 

 maxillae, jugals, and sometimes quadrato-jugals. To the quadrate region 

 belongs the squamosal ; to the mandible a dentary, and in Sauropsida a 

 splenial, coronoid, angular, and supra-angular. A tympanic bone under- 

 lying the tympanic cavity of the ear appears in Mammalia, and in many 

 Fish the opercular flap of integument covering the branchial cavity contains 

 opercular bones : see p. 93. 



The backbone is formed by the notochord and its sheath alone, in 

 Myxinoidei ; with the addition of neural arches in Petromyzon, and of 

 haemal arches as well in some Pisces (certain Elasmobranchii, chondrostean 

 Ganoidei}. In all other Vertebrata the notochord is constricted by 

 vertebral centra, and except in Pisces, some Amphibia and Reptilia, but 

 slight traces of it persist in the adult in the intervertebral regions. The 

 arches are developed in mesoblast independently of the notochordal sheath 

 in Ichthyopsida, in continuity with it in other Vertebrata. The neural arches 

 originate from a continuous right and left ridge in most instances ; they 



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