SKELETON. 65 
The elements of the branchial arches have the names, beginning above, pharyn- 
gobranchial, epibranchial, ceratobranchial and hypobranchial, the copul# 
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, J), 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 
2 
3 
\4 
“5 
6 
Fic. 64.—Branchial arches of (A) Heptanchus; (B), Chlamydoselache; and (C) Cestracion; 
A and C after Gegenbaur, B after Garman. , ceratobranchial; e, epibranchial; h, hyoid; 
hb, hyobranchial; he, hyoid copula; cbr, cardiobranchial (posterior copula); ~, 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 calcified 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 
