259 



afiFerent artery of the third arch and the other the afferent artery of 

 the fourth arch. Each of these four afferent branchial arteries tra- 

 verses its arch as a single vessel, but, in larvae, in the lower half of 

 each arch, a recurrent branch is sent downward and forward in the 

 arch to supply the ventral portion of the gill. In the adult these 

 recurrent branches are shown, in Allen's drawings, in the three 

 posterior arches only. 



From the antero-ventral surface of the afferent artery of the first 

 branchial arch, close to its point of origin and just as it turns back- 



Description of the accompanying Figure. 



Diagrammatic representation of the carotid and related arteries in Polyodon 

 spathula. aa I, II afferent arteries of first two branchial arteries, ahy afferent artery 

 of hyoidean arch. amd afferent artery of mandibular arch. cc common carotid. 

 cer cerebralis artery, da dorsal aorta, ea I, II efferent arteries of first two branchial 

 arches, ee external carotid, ehy efferent artery of hyoidean arch, emd efferent artery 

 of mandibular arch, hop hyo-opercularis artery, ic internal carotid. Ida lateral dorsal 

 aorta, om ophthalmica magna, op optic arteiy. 



ward to reach and enter its arch, a small vessel has its origin. This 

 vessel runs at first downward and forward, but almost immediately 

 curves backward and enters the hyoidean arch, where it can be traced 

 upward and backward beyond the proximal end of the ceratohyal, lying 

 always close to the internal lining membrane of the gill cover, parallel 

 and ventro-mesial to, but at a certain distance from, the afferent 

 pseudobranchial artery. This little vessel is evidently the afferent 

 hyoidean artery, strictly comparable to the artery in Amia (Allis, 

 1900 and 1908), and, as in that fish, greatly reduced because of the 

 total abortion of the hyoidean gill. Its position relative to the cerato- 

 hyal is strictly that of an aortic vessel of the arch, the artery lying 

 slightly external to the external edge of the ceratohyal. Slightly 

 internal to this artery is the afferent pseudobranchial artery, which 

 artery thus here has' the relative position of an efferent vessel of the 



17* 



