The pseudobranchial circulation in Amia calva. 1 |5 



The main part of the artery, on reaching the antero-mesial sur- 

 face cf the epihyal, turns downward, and, passing external to the 

 ligamentun» niandibulo-hyoideum, reaches the antcro-lateral surface of 

 the ceratohyal near the proximal end of that element. From there 

 it continues downward and forward, along the dorso-anterior edge of 

 the ceratohyal, and, beyond the distal end of that element, continues 

 along the lateral surface of the hypohyal to its anterior end, where 

 it turns inward around the anterior end of the element, and then 

 backward along its ventral surface. In this part of its course it 

 passes between the surfaces of insertion of the hyohyoideus and sterno- 

 hyoideus muscles, lying mesial to the former and lateral to the latter. 

 Having reached the posterior edge of this part of the ventral surface 

 of the hypohyal it turns sharply upward behind that element and 

 enters the base of the first branchial arch, lying ventral to the hypo- 

 branchial of the arch and mesial to the efferent artery of the arch, 

 near its anterior end. It then turns forward a short distance and 

 then laterally, and joins the anterior end of the efferent artery of the 

 arch. It is thus a direct continuation of that artery. In two of the 

 specimens examined, one cut in transverse and the other in horizontal 

 sections, it was impossible to tell, definitely, whether the artery did 

 or did not here have also a communication with the afferent artery 

 of the same ai'ch. In the third specimen, cut in longitudinal vertical 

 sections, there was certainly no trace whatever of such connection. 

 At the point where the artery turns sharply upward to enter the first 

 branchial arch it lies but a short distance from the afferent artery to 

 the gill cover, to be later described. 



This postero-ventral prolongation of the efferent pseudobranchial 

 artery of 12 mm larvae of Amia thus has, in its dorsal portion, the 

 same relation to the cartilage of the palato-quadrate arch that the 

 branchial arteries have to the cartilages of their respective arches; 

 that is it lies along the external surface of a part of the cartilage of 

 the mandibular arch , that external surface being naturally directed 

 backward because of the folding backward of the entire arch. In its 

 ventral portion the artery acquires relations to the hyoid arch, but it 

 there lies anterior to the cartilage of the arch and not posterior to 

 it, and it passes, in its course, external to the ligamentum niandibulo- 

 hyoideum. It thus has a position that it could naturally acquire by 

 simply slipping backward, off' the hind edge of the mandibular carti- 

 lages, as the cleft between these cartilages and the ' hyoid cartilages 

 was closed. In its extreme ventral portion the artery is the arteria 



8* 



