1^8 ALUS. [Vol. XVIII. 



and then directly forward, it gives off four branches, two on each 

 side. Both of these branches, on each side, have their origin 

 dorsal to the U-shaped ligament that connects the hind ends of 

 the large laminar processes of the third hypobranchials, and both 

 turn backward and laterally, one as the aft'erent artery of the 

 fourth arch, and the other as the afferent artery of the third arch. 

 The former is given oft' dorsal to the latter, the artery of the 

 fourth arch thus having its origin from the truncus distal to the 

 artery of the third arch. The artery of the fourth arch enters 

 at once the space enclosed in the concave surface of the distal end 

 of the fourth ceratobranchial, and, beyond the spoon-shaped end of 

 that bone, reaches the groove on its ventral surface. The artery 

 of the third arch runs backward and laterally along the mesial 

 surface of the large laminar processes of the third hypobranchial, 

 and passing dorsal to the posterior end of the process reaches the 

 ventral surface of the third ceratobranchial. 



The truncus, after giving off these two branches on each side, 

 continues forward, between the ventral portions of the opposing 

 mesial surfaces of the third hypobranchials, and, near the anterior 

 ends of those bones, gives oft', on each side, the aft'ereiit artery 

 of the second arch. This artery, on each side, turns laterally, 

 dorsal to the anterior end of the third hypobranchial, and passing 

 through the indentation in the postero-mesial edge of the shank 

 of the second hypobranchial, opposite the depressed region in 

 that surface, reaches the groove in the ventral surface of the 

 element. 



The main truncus then continues forward, between the opposing 

 mesial surfaces of the anterior ends of the second hypobranchials, 

 and, at the anterior ends of those elements, separates into two 

 parts, which turn laterally and backward, one on each side, and be- 

 come the afferent arteries of the first arches. Each of these 

 arteries reaches the groove on the ventral surface of the hypo- 

 branchial of its arch by passing through the indentation in the 

 postero-mesial edge of the element at about the middle of its 

 length. 



No indication whatever of an afferent branch to the hyoid arch 

 or to the gill cover could be found. 



