68 VERTEBRATE SKELETON 



hyostylic group, represented by most Elasmobranchs, Ganoids, and 

 Teleosts, the pterygoquadrate may articulate with the cranium, 

 but never fuses with it. The hyomandibula articulates with the 

 otic capsule at its upper end, its lower end with the quadrate part of 

 the pterygoquadrate, and is thus largely suspensorial (figs. 72, 74, 

 117, etc.). Several subdivisions of hyostyly are recognized. Auto- 

 styly, occurring in Dipnoi and Tetrapoda, has at least the pterygoid 

 part of the pterygoquadrate closely connected with the cranium, 

 while the hyomandibula (unless it be the stapes, p. 119) is reduced or 

 lost and is in no ways the suspensor of the jaws. 



In most Elasmobranchs, and here and there in higher classes, 

 cartilages occur in front of (external to) the mandibular arch (fig. 

 72, /) the morphology of which is uncertain. In Elasmobranchs 

 these labial cartilages lie just outside (morphologically anterior to) 

 the halves of the mandibular arch. The posterior group of these 

 consists of upper and lower halves, hinged hke the jaws, while in 

 front of this is only an upper labial on either side. 



These labial cartilages have been considered as remnants of visceral arches 

 which, in the ancestral Vertebrate occurred in hypothetical septa between gill 

 clefts anterior to the present mouth. This has Httle support, aside from their 

 position and the hinge of the posterior labials. Tabial cartilages occur as high 

 as the Crossopterygii and possibly the Amphibia. 



Ossification of the Skull 



In all Vertebrates above the Elasmobranchs the cartilage skull 

 is reinforced and more or less replaced by cartilage and membrane 

 bones. Since, in all living Vertebrates, cartilage appears before 

 membrane bones, and as the preceding paragraphs give a cartilage 

 framework on which to build, the cartilage bones are described here 

 before the others, although in ontogeny (probably in phylogeny), 

 membrane bones appear before any ossifications in the cartilage. 



Names were first given to the bones of the human skull, from which they 

 have been transferred with more or less success to the lower groups, with such 

 changes as necessity has caused. Some single bones in the adult human skull are 

 represented by two or more separate elements in some of the lower classes, while 

 there arc numerous instances of piscine, Amphibian or reptilian bones which 

 are absolutely lacking in man. There is a greater difference between the skulls 

 of fishes and Tetrapoda than between the lowest Amphibian and that of man. 

 The bones of the most primitive Amphibians, the Stegocephals, are made the 

 basis of the following account. 



