86 



VERTEBRATE SKELETON 



arches are posterior to the cranium and are not connected with it, 

 but some or all may be attached to the anterior vertebrae in skates. 

 The anterior branchial arches are most complete, there being a 

 tendency to reduction, modification, and fusion in the posterior. 

 Each arch typically has four elements (p. 65), the two halves of 

 each arch being connected by a basibranchial, but it is uncertain 

 with whether, morphologically, these copulae.He between or alternate 

 the arches. Very frequently the successive copulae fuse to a larger 

 plate, and the last of the series may be very large, and, when lying 

 just ventral to the heart, is called a cardiobranchial cartilage (fig. 



69, A, C). Each arch supports a number of 

 cartilage branchial rays which extend from 

 epi- and ceratobranchials as supports into 

 the branchial septum. Usually fiUform, 

 they are sometimes branched or plate-like, 

 and are most numerous in skates. A num- 

 ber of Elasmobranchs have extrabranchial 

 cartilages, above and below, and farther out 

 in the septum than the arch, which form an 

 additional support for the septum (fig. 89) 



The modifications of the hyoid arch are 



important. Notidanids (fig. 93) are most 



ex, extrabranchiais; pb, pha- primitive, the hyomandibula being small, 



ryngobranchial. , . , . , , ^. 



restmg agamst the otic capsule and articu- 

 lating ventrally with the hyale only, its connexion with the man- 

 dibular arch being only by hgament. Most Elasmobranchs have 

 the ventral end of the hyomandibula larger and articulating 

 with, not only the ceratohyoid, but with the mandibular arch as 

 well, thus forming part of the suspensor of the jaws. This reaches 

 its extreme in Torpedo. In Raise (fig. 88) the hyomandibula is 

 purely suspensorial, the hyale being separate from it and articu- 

 lating directly with the otic region. Hyomandibula and ceratohyal 

 bear gill rays. The hypohyals of the two sides are connected by a 

 basihyal which often has an anterior hngual process, sometimes 

 interpreted as indicating a former copular connexion of mandibular 

 and hyoid arches, similar to the connexion of hyoid and branchial 

 arches in some species. 



The mandibular arch (pterygoquadrate and Meckelian cartilages) 

 forms the functional upper and lower jaws, each with teeth, usually 



Fig. 89. — Branchial arch 

 Gi Heterodontiis (Daniel, 15). 

 br, branchial rays; cb, cerato- 

 branchial; eb, epibranchial; 



