intertemporal hyomandibula-- 

 sclerotrc plate 

 porietal 

 supraorbital 

 nasal capsule^ „□;□! 

 medial rostral, 

 lateral rostral^ 



extrascapular series 

 posttemporal 



suprapreopercle 



Figure 5-1. Lateral view of the head skeleton of the salmon. 



The myodome is a cavity between the base of the cranium 

 and the parasphenoid. The prootics meet above this space, 

 and form part of its lateral wall. This cavity contains the poste- 

 rior parts and the areas of origin of the external, internal, 

 and inferior rectus muscles of the eye. The myodome is 

 small in the cod and does not extend below the level of the 

 basis cranii. The salmon is peculiar in al.so having an ante- 

 rior myodome, a midline chamber extending forward nearly 

 to the tip of the rostrum (Figure 5-5). 



The hyomandibula articulates with the sphenotic and 

 pterotic parts of the otic capsule wall. It has an opercular 

 process extending posteriorly, as well as a process extending 

 out to the preopercle. The body of the bone is perforated 

 by the hyomandibular branch of the seventh cranial nerve. 

 The distal end articulates anteriorly with the symplectic 

 and posteriorly with the interhyal; both of these joints are 

 synchondroses. The interhyal is a small bony rod between 

 the hyomandibula and the posterior end of the triangular 

 ceratohyal posterior. 



The ceratohyal is divided into anterior and posterior 

 parts; in the cod or perch (Figure 5-3 D), these are joined 

 internally by straps of bone, which meet along a deeply 

 intcrdigitating suture line. The hypohyal is made up of dor- 

 sal and ventral parts, synchondrally joined. The hypohyals 

 join at the midline where the dorsal portion is attached to 

 the midline basihyal. A urohyal extends ventroposteriorly 

 as a thin midline plate. In the salmon there is an anteriorly 

 projecting, tooth-bearing, entoglossal attached to the basi- 

 hyal. 



Behind the hyoid arch are four branchial arches. With 



the exception of the first, each of these has four segments; 

 from below upward these are hypobranchial, ceratobran- 

 chial, epibranchial, and pharyngobranchial. In the salmon 

 the first arch lacks a pharyngobranchial but has a large 

 suprabranchial, which attaches to the prootic just above 

 the internal cartotid foramen. Each of the four epibranchials 

 has a dorsal process on its upper end. A fifth pair of cerato- 

 branchials is present. 



EMBRYOLOGiCAL DEVELOPMENT In the teleost. Cartilage 

 first appears at either side of the notochord, forming the 

 parachordal rods (Figure 5-6). Anteriorly these rods turn 

 outward: posteriorly they are flattened and extended dor- 

 solaterally by the rudiments of the occipital arch. Meckel's 

 cartilage, the ceratohyal, and hyomandibula are present. 

 Well anterior to the tips of the parachordals a pair of 

 trabeculae chondrify. Polar cartilages appear between the 

 ends of the trabeculae and parachordals. Distal to the 

 hyomandibula, but joined with it by a procartilaginous 

 connective, is the sympletic. The first and second cerato- 

 branchials are present and also an intermandibular carti- 

 lage between the tips of Meckel's cartilages. 



The trabecula and parachordal are joined by the polar 

 cartilage. Anterior and posterior centers appear in the otic 

 capsule; these are connected by anterior basicapsular and 

 basivestibular commissures, respectively, with the para- 

 chordal. A posterior capsular commissure soon forms behind 

 the glossopharyngeal nerve. 



An occipital arch develops and the anterior ends of the 

 trabeculae become joined by a plate. The lateral commis- 



GNATHOSTOME FISHES • 107 



