428 C. H. DANFORTH 



From the third gill there may or may not be a recurrent 

 vessel. When present it is comparable to the others in its gen- 

 eral relations but has a rather limited distribution. 



The fourth efferent artery, so far as my experience goes, invari- 

 ably gives off a recurrent vessel which has median and lateral 

 branches. From the former are derived the anterior vasa of 

 the ventral aorta (fig. 3 a.mhh.') and other small twigs. The 

 lateral branches, possibly to be considered collectively as pos- 

 terior remnants of the . lateral hypobranchial, are anterior and 

 posterior as regards their direction. The former {Ihh.'") is small 

 and may anastomose anteriorly with a similar vessel from the 

 third gill when there is one present. The main trunk of the 

 posterior branch {Ihb.'"^), which is larger and more diffuse in 

 its distribution, gains the median aspect of the fifth cerato- 

 branchial and runs back for a considerable distance. Its ter- 

 minal twigs reach those of the vessels from behind the heart 

 (fig. 12) and may even anastomose with them, but any connection 

 here is probably only secondary. In distribution, but not in 

 origin, it suggests the dorsal median hypobranchial artery of 

 Silvester's account. 



The restricted distribution of the hypobranchial arteries of 

 Polyodon is probably to be correlated with the fact that the 

 hypobranchial region as a whole is greatly reduced and pushed 

 far forward so that the direct connection between this and the 

 pectoral region is unusually slender, consisting for a considerable 

 distance of little but cartilage and tendons. The great varia- 

 bility of the minor vessels on the other hand is a characteristic 

 shared by fishes in general. But that there should be no coron- 

 ary artery in connection with this system is interesting and 

 apparently unique. Allen (op. cit., pi. 3, fig. 5) shows a vessel 

 arising in the second gill and indicated as 'coronary artery.' 

 This figure was made to show lymphatics, however, and is almost 

 certainly incorrect so far as the artery is concerned. The vessel 

 in question is either the commissural or the artery of supply to 

 the sternohyoideus; and Allen himself states in the text that 

 the coronary artery "comes from the fourth right efferent bran- 

 chial artery and approaches the heart from the rear." This 



