SKELETON. 65 



The elements of the branchial arches have the names, beginning above, pharyn- 

 gobranchial, epibranchial, ceratobranchial and hypobranchial, the copulae 

 being the basibranchials. The elements of the hyoid are correspondingly, epi-, 

 cerato-, 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 



FIG. 64. Branchial arches of (A ) Heptanchus; (B), Chlamydoselache; and (C) Cestracion; 

 A and C after Gegenbaur, B after Garman. c, ceratobranchial; e, epibranchial; h, hyoid; 

 hb, hyobranchial; he, hyoid copula; cbr, cardiobranchial (posterior copula); p, pharyngo- 

 branchial; 1-7, branchial arches. 



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 gill arches. 

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

 gnathostomes, and ignores the peculiarities of the cyclostomes which are summar- 

 ized below. 



In the elasmobranchs and cyclostomes the skull is cartilaginous 

 throughout life, or at most is calcined cartilage, without sharp division 

 into separate elements. In the higher vertebrates the cartilage is sup- 

 plemented or almost entirely replaced by bone which may be of the 

 two kinds, cartilage bone and membrane bone (p. 42), the distinctions 

 between which must constantly be kept in mind in tracing homol- 

 ogies in the different classes. The membrane bones are usually 

 derivatives of the deeper or dentinal layer of scales or teeth which have 

 fused together (fig. 65) and have sunk to a deeper position, coming 

 5 



