THE HEART AND ARTERIES OP POLYODON 427 



bly more, of the thyreoid and terminate as the nutrient arteries 

 of the branchiomandibularis (m. bmd.) which they reach by pass- 

 ing between its two tendons of insertion to gain the ventral side 

 of the muscle. But while still dorsal to the branchiomandibu- 

 laris several minor branches are given off. One of these (a.hb.y.) 

 is very constant in its occurrence. It arises near the median 

 line and runs laterad beneath and close to the insertion of both 

 the sternohyoideus and transversalis I. It here comes into 

 contact with the hyoidean artery, twice crossing its dorsal sur- 

 face, but apparently never anastomoses with it. Its final dis- 

 tribution is over the hypobranchial cartilage and the surrounding 

 connective tissue. 



In these vessels what may represent lateral, medial and com- 

 missural elements can, perhaps, all be recognized, but this is 

 a point which should not be pressed too strongly, for the vessels 

 are all variable and it is doubtful if they have much significance 

 for comparative purposes. The two lateral branches could to- 

 gether be interpreted as constituting a lateral hypobranchial 

 artery, and the fact that the anterior division sends a twig to 

 the thyreoid is in agreement with that vessel in the elasmo- 

 branchs as described by Ferguson ('11). Whether or not the 

 posterior branch ever effects a communication with the arteries 

 from the third or fourth gills I cannot say. Apparently it usually 

 does not, although in some cases the terminal twigs come very 

 close to one another. The portion of the recurrent vessel be- 

 tween the 'lateral hypobranchial' and the middle line is in the 

 correct position for a commissural artery and the rest of the 

 vessel {a.mlib.) is the median hypobranchial. It is in accordance 

 with teleostean conditions, according to Silvester ('04) and 

 Gudernatsch ('11), for the median hypobranchial to supply the 

 thyreoid. The supply of the median part of the thyreoid, when 

 this vessel is lacking, has not been fully determined, but appar- 

 ently the gland then draws exclusively on the lateral hypo- 

 branchials. The median hypobranchial is not produced back- 

 ward beneath the aorta as it is in almost every fish thus far 

 described, but all the derivatives of the second afferent artery 

 are confined to the region anterior to the third gill. 



