Nos. IAND2.] ANATOMY OF SCOMBER SCOMBER. 123 



groove extends forward into the bone and ends blindly. Toward 

 its hind end the groove widens and then disappears, the bone at 

 this point being slightly wider than it is immediately in front of 

 it. Opposite the hind end of the groove the bone narrows some- 

 what abruptly, and then ends in a relatively long, cylindrical and 

 bluntly pointed tip of cartilage, which turns gradually downward 

 and, at its extreme hind end, lies in an almost vertical position. 

 The distal end of this tip of cartilage lies in, and gives attachment 

 to, the median, posterior portion of a U-shaped tendon, from 

 which the obliqui ventrales of the fourth arch arise. The anterior 

 ends of this U-shaped tendon are inserted, on each side, on the 

 ventral edge of the third hypobranchial. The truncus arteriosus 

 reaches the ventral surface of the basal line immediately in front 

 of the hollow of the U, thus lying immediately ventro-anterior to 

 the curved cartilaginous end of the third basibranchial. 



The posterior portion of the third basibranchial, and the related 

 part of the truncus arteriosus, are both enclosed between the 

 scooped-out anterior ends of the hypobranchials of the third pair 

 of arches. These third hypobranchials do not articulate with the 

 main, central piece of the basal line, but do articulate with the 

 anterior edge of a separate median piece of cartilage that lies 

 immediately dorsal to the cartilaginous end of the third basi- 

 branchial. The fourth ceratobranchials also articulate with this 

 median cartilaginous piece, and this, together with its general 

 position, seems to indicate that it represents the fused hypo- 

 branchials of the fourth arch rather than a part of the basal line, 

 and I have so considered it. 



The Fourth Basibranchial (BB*) is a small piece of carti- 

 lage that lies between the adjoining anterior ends of the fifth cera- 

 tobranchials, the anterior ends of those ceratobranchials lying be- 

 tween the scooped-out anterior ends of the fourth ceratobranchials, 

 slightly behind, and slightly dorsal to the median cartilaginous 

 piece with which those ceratobranchials and the third hypo- 

 branchials articulate. 



3. Branchial Arches. 



The first branchial arch (Figs. 29 and 30) contains five pieces, 

 which seem to be the five elements said by van W^ijhe to belong to 

 a complete and perfect arch (No. 4, p. 662). That piece, how- 



