On the Anatoipy and Phylogenetic Position of Polypterus. 389 



somewhat and the lower jaw slightly pushed forward. At the same 

 time the spiracle is closed. 



The innervation is from a branch of the mandibular nerve, sepa- 

 rating off near its origin. 



Between these two muscles and the skull wall is situated the 

 spiracular passage. 



The Retractor hyomandibularis and opercularis form a fairly stout 

 sheet (fig. 3) , arising from the postero - external process of the Pe- 

 trosum especially from the ridge which traverses it. The anterior 

 portion of the sheet is stouter and as the Retr. hyom. is attached to 

 the inside of the upper arm of the bent hyomandibular above the 

 articulation of the operculum. The posterior portion of the sheet of 

 muscle passes downwards and backwards attaching itself to the inner 

 surface of the operculum while many of its fibres continue on as the 

 intrinsic muscles of the opercular mantle. 



As belonging to this same bundle primitively, may be considered 

 a small muscle at the angle of the jaw in the substance of the liga- 

 ments which bind the hyomandibular, quadrate and stylohyal to the 

 lower jaw (fig. 3). It is however also possible that it may represent 

 an Adductor arcus branchialis. These muscles are antagonistic to 

 those previously described. By their action the gill cavity is closed. 

 The Retractor hyomandibularis in conjunction with the small muscle 

 at the angle of the mouth may help to depress the jaw. 



Innervation is from the opercular branch of the hyoid branch of 

 the Facialis. 



Intermaxillaris anterior. 



If the jugular plates and skin be removed from the lower jaw an 

 intermaxillaris anterior is seen arising on either side from a median 

 raphe (fig. 4). Its fibres pass horizontally externally and anteriorly 

 to the ligaments along the edge of the mandible extending from the 

 1st fifth to the 3rd fifth of its length. 



Innervation is from a branch of the mandibular nerve (R. m. i.) 

 which leaves the lower jaw at half its length. 



The intermaxillaris posterior arises from the same median raphe 

 a portion of its fibres lying anteriorly under the intermaxillaris anterior. 

 Its fibres run approximately at a right angle with those of the an- 

 terior. Their attachment is mainly to the ceratohyal but partially also 

 to the jugular plate at its posterior edge. Some fibres are continued 

 on as intrinsic muscles in the mantle flap. 



