MORPHOLOGY OF TELEOSTEAN HEAD SKELETON. 541 



epihyal (fig. 8, hy.) has lost its bar-like character in the 

 upper half and become converted into a square plate, and 

 thus resembles the same element in Syngnathus (McMurrich, 

 83, p. 631). 



The symplectic is unchanged, but the hyomandibular (fig. 8, 

 h7jm.) has grown rapidly in such a way as to transform the 

 triangle of the previous stage into a rectangle, whose long axis 

 is parallel to that of the skull, and whose attachment to the ear 

 capsule is now horizontal. At each end of this attachment 

 the cartilage has swollen somewhat to form an articular head, 

 whilst the intervening portion remains unchanged. Thus at 

 this stage the hyomandibular articulation is beginning to 

 depart from that simple elongated type which it exhibited in 

 the earlier stages, and which is retained in the adult state in 

 many Teleosts, e. g. salmon ; and to take on that two-headed 

 type of articulation seen in the pike, and so commonly 

 possessed by Acanthopterous fishes. The condition at this 

 stage might with fairness be compared to that described by 

 Gegenbaur for Alepocephalus (78, p. 14). There the double- 

 headed type is indicated, but the simple type is not yet quite 

 lost. 



Owing to these growth processes, tlie hyomandibular nerve 

 foramen lies in the centre instead of near the anterior border 

 of the hyomandibular. 



The quadrate (fig. 8, qu.) has also grown, but is still 

 triangular with the articular head for the mandible, and 

 the palatine, and the metapterygoid processes in the same 

 relative positions as before. The latter pi'ocess is propor- 

 tionately smaller, but this is due to growth of quadrate rather 

 than to actual reduction of the process, for its extremity is still 

 opposite the insertion of the stylohyal. In young Graste- 

 rosteus spinachia the extremity of the metapterygoid 

 is removed far in front of this point, owing to the continued 

 elongation of symplectic and ethmoid. The mouth and 

 its associated parts are thus carried much further forwards 

 from the eye than in G. aculeatus. 



The palatine process I'etains its uniformity of thickness, 



VOL. 45, PART 4, NEW SERIES. 



