96 



VERTEBRATE SKELETON 



cartilage is continuous at first, dividing later into cerato- and hypo- 

 hyals, the hypohyals of the two sides being connected by a basihyal 

 copula. 



The branchial arches (fig. 102), normally five in number, arise as 

 continuous bars, segmenting later into the separate parts of the adult 

 ■ — usually four (pharyngo-, epi-, cerato- and hypobranchial) in the 

 three anterior arches, each of which is more or less ossified, the dorsal 

 parts of each arch being the most variable. The fourth arch usually 

 lacks the pharyngobranchial; the ceratobranchial is the most con- 

 stant part of the fifth arch. 



In all Teleostomes except Chondrostei the pterygoquadrate and 

 its associated bones are intimately associated with the cranium, and 



the functional jaw is composed 

 of dermal bones, usually a pre- 

 maxilla at the tip of the snout, 

 followed by a maxilla on either 

 side. Even in Acipenser (fig. 

 107) a membrane bone called a 

 maxilla occurs on the pterygo- 

 quadrate, but it is more like 

 a dermopalatine. The quadrate 

 part ossifies as a separate quad- 

 rate bone which bears the artic- 

 ular surface for the lower jaw. 



The following bones may 

 occur in connexion with the 

 pterygoquadrate (fig. 74) : At 

 the extreme tip an autopalatine 

 ossifying in the cartilage, and associated with a true (dermal) palatine, 

 the compound bone usually articulating with the ectethmoid, occasion- 

 ally extending farther forwards. The palatines are followed by two 

 membrane bones, a more dorsal entopterygoid and a more ventral 

 ectopterygoid, while farther back the cartilage ossifies above as 

 metapterygoid ; below is the quadrate. Some of these five bones are 

 sometimes absent; Siluroids, for example, having but palatine, meta- 

 pterygoid and quadrate. 



The posterior end of Meckel's cartilage ossifies as an articulare, 

 said to fuse with a membrane bone, the goniale, which is probable, 

 since in Tetrapoda the adult articulare often has such double origin. 



Fig. 102. — Branchial arches of Acipenser 

 sturio (van Wijhe, '82). Bone, black; car- 

 tilage stippled, b, basibranchial; cb, cerato- 

 branchial ; e, epibranchial ; hb, hypobranchial, 

 i, lower pharyngobranchial; p, pharyngo- 

 branchial; s, suprapharyngobranchial. 



