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C. H. DANFORTH 



aorta. At the level of the third hypo-branchial cartilage it 

 divides into the two afferent arteries. One {a.hr.a. 3) enters the 

 third gill in the same way as those described above, while the 

 other {a.hr.a. 4) reaches the fourth in what at first sight appears 

 a very unusual manner. It ascends in an obhque groove on 

 the lateral face of the second copula (fig. 5) to gain the floor of 

 the mouth where it is separated from the oral cavity only by 

 the mucosa and the shghtest amount of subjacent tissue (fig. 

 6, A). It crosses dorsally the anterior end of the ventral carti- 

 lage of the fourth branchial arch and then turns down in a groove 

 on the medial and posterior aspect of that cartilage to enter the 

 gill along the m. obhquus ventralis IV, thus coming into corre- 



c. cer. 5 c. cer. 4 



c. CO. 3 



c. CO. 2 



a. br. a. 4 a. br. a. 3 a. an. 



a. br. a. 4 



Fig. 5 The origin and proximal relations of the fourth afferent branchial 

 artery. 



spondence with all the more anterior afferent arteries. Caudad 

 to the groove for the artery there is a ligament binding the branch- 

 chial cartilage to the second copula, and this is interpreted by 

 Bridge (79) and Van Wijhe ('82) as a second articulation. Such 

 an interpretation seems justifiable, and, if it be correct, brings 

 Polyodon into accord with Amia and Acipenser, in so far as 

 this point is concerned. In the third and fourth arches of Amia 

 the hypobranchial articulates with the copula near the floor 

 of the mouth and with its ventral keel at a deeper level (fig. 6, B). 

 The artery passes through the enclosure thus formed. With 

 Polyodon the same condition exists in the third arch, but in 

 the fourth, where the hypobranchial has disappeared or lost 



