SKELETON 73 



the hyoid are correspondingly, epi-, cerate-, and hypohyal. These parts lie in 

 the medial ends of the gill septa, medial to the aortic arches. 



Other cartilages, which seem to be of less morphological importance, occur 

 in the same region. Among these are the labial cartilages (fig. 67, /)> usually 

 two above and one below, which lie in front (outside) of the cartilages of the 

 mandibular arch of sharks,, and in a modified form as high as some of the 

 ganoids. By some they are regarded as remnants of visceral arches of the 

 preoral region. In the branchial region of the elasmobranchs a variable number 

 of extrabranchial cartilages may occur, small bars external and parallel to the 

 upper and lower ends of the giU arches. 



The foregoing sketch of the chondrocranium is based on conditions in the 

 gnathostomes, and ignores the peculiarities of the cyclostomes which are 

 summarized below. 



In the elasmobranchs and cyclostomes the skull is cartilaginous 

 throughout life, or at most is calcified cartilage, without sharp divi- 

 sion into separate elements. In the higher vertebrates the cartilage 

 is supplemented or almost entirely replaced by bone which may be 

 of the two kinds, cartilage bone and membrane bone (p. 47), the 

 distinctions between which must constantly 

 be kept in mind in tracing homologies in the 

 different classes. The membrane bones are 

 usually derivatives of the deeper or dentinal 

 layer of scales or teeth which have fused to- 

 gether (fig. 73) and have sunk to a deeper posi- y^^ 73.-V^mer of 

 tion, coming into close connexion with the ele- 25 mm. Amblystoma 

 ments derived from the cartilage skull, in some showing the bone devel- 

 cases replacing considerable of it. The carti- °^^l^^ ^teeth'°° °^ 

 lage bones arise by an ossification of the car- 

 tilage. Even in the sturgeons the chondrocranium is complete, the 

 membrane bones being superficial and not intimately connected with 

 the deeper parts. 



The names of the bones are largely based on the terminology of human 

 anatomy. In many cases what appears as a single bone in the human skull is 

 represented by several bones in the young and in the lower vertebrates. In 

 these cases the bones in the lower forms are usually given names which indicate 

 their relation to the human bones or to the part or region in which they occur. 

 Dermal bones are apparently the older, phylogenetically, but for convenience 

 the cartilage bones are considered first. 



The chondrocranium shows several centres of ossification, but 

 only those giving rise to distinct bones are considered here.^ The 



' Basj- and presphcnoid, for example, arise each from two centres, but in all vert^• 

 brates the resulting bones are unpaired. 



