260 



arch. A short basal portion of the afferent hyoidean artery is shown 

 in Allen's drawings of the adult, but it is not identified or named 

 by him. 



The only other afferent artery in Polyodon is the afferent pseudo- 

 branchial artery, just above referred to. This artery, which certainly 

 is, in PolyodoD, as it is in Amia and teleosts, the afferent artery of 

 the mandibular arch, has, even in my youngest specimens, already 

 wholly lost its connection with the truncus arteriosus and acquired 

 an origin from the anterior branch of the efferent artery of the first 

 branchial arch, near, but not at, its ventral end. In Allen's drawings 

 of the adult it is shown as a direct antero-ventral prolongation of 

 the latter artery. From its point of origin it runs forward until it 

 reaches the postero-mesial surface of the hypohyal, where it turns 

 laterally and backward along the ventro-mesial (external) edge of that 

 element. Beyond the hypohyal, the artery continues onward along 

 the ventro-mesial (external) edge of the ceratohyal, sending branches 

 to a large muscle of the region, this muscle apparently being the 

 homologue of some part of the constrictor muscles of Vetter's (1878) 

 descriptions of Acipenser. As the artery approaches the posterior 

 (proximal) end of the ceratohyal it turns upward across the antero- 

 lateral surface of that element, and then across the lateral surface of 

 the symplectic, passing between the latter element and the quadrato- 

 symplectic hgament of Bridge's (1879) figures. It then turns upward 

 forward and mesially along the ventro-anterior edge and the internal 

 surface of the hyomandibular, to reach and enter the pseudobranch. 

 As it turns upward to cross the lateral surface of the symplectic it 

 sends a branch backward internal to the distal end of the hyomandi- 

 bular, this branch quite unquestionably being the homologue of the branch 

 of similar origin described by me in Amia (Allis, 1900, p. 114) and 

 by Maurer (1888, p. 215) in Salmo, and representing, as in those 

 fishes, a remnant of a commissure that in younger larvae undoubtedly 

 connected the hyoidean and mandibular aortic arches. 



The afferent pseudobranchial artery of Polyodon is thus seen to 

 have relations to the ceratohyal that are, in some respects, quite 

 different from those of the corresponding artery in Amia and in 

 teleosts. In Amia (Allis, 1900) the artery runs forward ventral 

 (external) to the distal end of the hypohyal to reach the antero- 

 lateral surface of that element, and from there runs upward and 

 backward along the antero-lateral surface of the ceratohyal, near its 

 dorsal (internal) edge. In Scomber (Allis, 1903) the artery perforates 

 the hypohyal, between the two ossifications of the element, and so 

 reaches the antero-lateral surface of that element and then the cor- 



