128 ALUS. [Vol. XVIII. 



branchial lying between the dorso-mesial surfaces of the larger 

 laminar processes of the hypobranchials, and that part of the 

 ventro-mesial surface of the body of each hypobranchial that lies 

 dorsal to that process. The ventro-mesial edge of the large lami- 

 nar process of each element is nearly horizontal in position, lies 

 ventral to the truncus arteriosus and not far from the correspond- 

 ing edge of the hypobranchial of the opposite side of the head. 

 The afferent artery of the second arch turns outward and back- 

 ward dorsal to the anterior end of the third hypobranchial, between 

 it and the ventral surface of the basal line. The arteries to the 

 third and fourth arches are given off immediately in front of the 

 turned-down posterior end of the third basibranchial, and run 

 outward and backward dorsal to the projecting hind end of the 

 laminar process of the third hypobranchial. 



The Fourth Hypobranchial seems, as already stated, to be 

 fused with its fellow of the opposite side of the head, and to be 

 represented in that small, median piece of cartilage that lies imme- 

 diately dorsal to the cartilaginous hind end of the third basi- 

 branchial. It gives articulation to the proximal ends of the 

 third hypobranchials and to the distal ends of the fourth cerato- 

 branchials, as already described. 



The First Three Ceratobranchials {CB, I-III) are prac- 

 tically alike in shape, but they decrease more or less in length 

 from the first to the third. They are all slightly curved, semi- 

 cylindrical rods of bone, the ventral surfaces of which are deeply 

 grooved their full length. The third ceratobranchial is somewhat 

 more curved than the second, and the second somewhat more than 

 the first. The hollows of the curves are directed orally, that is, 

 toward the branchial chamber. Both ends of all three elements 

 are capped with cartilage. The distal end of the third cerato- 

 branchial has usually, on its postero-mesial corner, a small process 

 which takes part in the formation of the articular end of the piece, 

 thus giving it an articular surface somewhat larger than those of 

 the other two bones. 



The Fourth Ceratobranchial {CB, IV) has almost exactly 

 the length of the second, and is accordingly somewhat longer than 

 the third. It is more curved than the anterior bones, and is also 

 somewhat flattened. The distal third, approximately, of the bone 



