THE HEART AND ARTERIES OF POLYODON 437 



where the hyomandibular nerve enters it and through this ante- 

 rior part of the channel the artery again escapes from the cranial 

 wall, emerging dorsal to the m. protractor hyomandibularis be- 

 tween its two slips of origin. As it leaves the canal it gives off 

 a second branch. This, together with the first, probably repre- 

 sents the equivalent of the hypoopercular artery of Silvester's 

 descriptions (for teleosts) or the hypoopercular and external 

 carotid of Amia (Allis). 



The posterior branch is here called the hyoopercular (a.hyo.) 

 and is likewise designated by Allis in his recent paper. Leaving 

 the facial foramen along with the nerve, it ascends abruptly 

 the posterior aspect of the hyomandibular bone, gives off a 

 large muscular branch to the adductor hyomandibularis (fig./ 

 16 m.adh.) and assumes a superficial position along the insertion 

 of this muscle. Several small branches cross over to the ante- 

 rior side of the hyomandibular bone and a very long one runs 

 back under the operculum (oper.) to the inner side of the oper- 

 cular flap. The main vessel, still following the hyomandibular 

 nerve, traverses a groove in the distal end of the cartilage from 

 which it sends a second branch to the opercular flap and then 

 passes down under the branchiostegal ray (6r.) to be distributed 

 posterior and medial to the area supplied by the facial artery. 

 For a further discussion of this artery the reader is referred to 

 Allis's paper (1. c, p. 286). 



The second branch of the external carotid immediately sepa- 

 rates into two divisions. In the specimen described by Allis (I.e., 

 p. 285) they arose separately from the main trunk. One runs 

 laterally beneath the ep'thelium in front of the spiracular cleft 

 and along the anterior face of the hyomandibular bone. The 

 other divides into external and internal branches. The former 

 (a.com.) ramifies over the cartilage around the spiracular cleft 

 and diverticulum and supplies a nutrient branch to the anterior 

 division of the protractor muscle. The latter (a.fg.) passes 

 through the cartilage into a large 'fat-space' (the F-shaped groove 

 of Bridge) beneath the frontal bone and medial to the hyoman- 

 dibular articulation where it breaks up into small twigs. 



