The Latero-Sensory Canals and Related Bones in Fishes. 463 



tube, the 2iid. of the line, is given off as the canal passes from the 

 nasal into the ethmoid, or in the hind end of the nasal,, but there is 

 no tube between the ethmoid and the frontal. In the nasal there is 

 one sense organ, and as there is no organ in the ethmoid it is evident 

 that the canal is secondarily enclosed in this latter bone, and that the 

 bone is a purely primary ethmoid. Having- entered the frontal, the 

 canal runs backward in a curved course until it reaches the base of 

 what Traquair calls the external anterior ang-le of the bone, where it 

 lies at a deep level, near the internal, ventral surface of the bone. 

 There it first turns, in a rounded angle, upward and mesially, that is 

 upward, toward the rigiit and even slightly forward, and then soon 

 turns laterally (to the left) at an acute angle. At the acute angle 

 it gives off primary tube No. 3, this tube running mesially (to the right) 

 in the frontal, to join its fellow of the opposite side in the suturai 

 line between the two frontals. Posterior to this third tube the canal 

 runs laterally and forward and soon bifurcates, one long branch running 

 forward and laterally to the lateral edge of the anterior portion of 

 the frontal, and the other laterally and backward to join the main 

 infraorbital canal. The long antero-lateral branch is, like the fourth 

 tube on the rigiit side of the head, one half of the double tube formed 

 where the supraorbital and infraorbital canals anastomose, the other 

 half of this double tube arising from the main infraorbital between 

 the frontal and postfrontal bones. Two sense organs are found in 

 the frontal part of the canal, one lying anterior to the point of origin 

 of tube No. 3, and the other opposite or slightly posterior to that point, 

 exactly as on the right side. 



In Pleuronectes , on the right side, Cole and Johnstone find four 

 supraorbital organs in the frontal, where I find but two in Hippo- 

 giossus; and yet there are apparently the same number of primary' 

 tubes in the two fishes. Whether this means that I have wholly 

 failed to find one or more sense organs here, or that the three 

 posterior of the four organs shown by Cole and Johnstone are simply 

 slightly differentiated parts of a single large sense organ, I can not 

 positively decide. The curious innervation shown in Pleuronectes, 

 however, suggests that there is here. but a single organ, instead of 



