SKULL — TELEOSTS ■ IO7 



quadrate appears first, the pterygoid process growing forwards from this, the 

 quadrate part always remaining the larger. Meckel's cartilage ofTers nothing 

 of importance. In the precartilage stage the hyoid arch consists of three ele- 

 ments on a side (hyomandibula, symplectic and the upper part of the hyale), 

 the hyomandibula forming around the hyomandibular nerve, the foramen often 

 persisting in the adult. The gill-arches appear in order from in front backwards, 

 the last being the most rudimentary, the fourth frequently lacking the hyo- 

 branchial, the fifth being even more reduced. At first hyoid and gill arches 

 are connected by a continuous copula which segments later into an anterior 

 basihyal, prolonged forwards as an (often separate) entoglossal. followed by a 

 varying number of basibranchials. 



A pecuKarity of many adult Physostomes (Siluroids excepted) 

 and Acanthopterygians, is an eye-muscle canal (myodome) which 

 contains two of the rectus muscles of the eye. It is formed early 

 (before much ossification) and seems to have arisen as a result of 

 reduction of the medial wall of the orbit, forcing the muscles to 

 obtain another origin. The muscles grow back between brain and 

 basal plate, sometimes extending so far as to pass out of the cranial 

 cavity behind, their ends lying ventral to the basal plate and between 

 it and the developing parasphenoid. In typical conditions proc- 

 esses from the two prootics meet in the middle Une, roofing the 

 anterior part of the canal, the posterior being excavated in the basi- 

 occipital and floored by the parasphenoid. The tube may be 

 divided, an upper part containing the hinder part of the external 

 rectus, the lower that of the internal rectus muscle. Usually the 

 canal extends to the occipital region and may be open behind or end 

 blindly. 



Some of the most interesting modifications of the skull occur in Pleuronectids. 

 At first these fishes are bilaterally symmetrical and swim in the normal position. 

 Then the young turn on one side, lying hereafter with that side downwards. 

 This change of position results in the transfer of the eye from the lower to the 

 upper side, twisting the bones and destroying the primitive symmetry of the 

 anterior part of the cranium, the posterior part remaining nearly normal 

 (fig. 114). 



The four occipitaha usually ossify in the base of the cranium. 

 The supraoccipital has a marked dorsal crest, often produced back- 

 wards as an occipital spine to which the dorsal trunk muscles are 

 attached. This bone usually separates the parietals when these are 

 present, and extends forwards to the frontal, but sometimes 

 the parietals overlap the supraoccipital, occasionally meeting in the 



