SEA- FISHERIES LABORATORY. 153 



classic memoir,* the facts and conclusions of which we 

 can fully confirm, was enabled by an exhaustive examina- 

 tion of the skull and lateral line system to map out the 

 exact course followed by the head before it reached its 

 present remarkable form. 



The first difficulty in the solution of the problem is 

 the position of the anterior extremity of the dorsal fin. 

 If this occupies the mid-dorsal line of the head, then it is 

 obvious that the left eye must have actually passed 

 through the substance of the head to reach the ocular side. 

 This supposition, absurd as it may seem to us now, was in 

 fact believed by such an observer as Steenstrup. But the 

 anterior extremity of the dorsal fin is not situated in the 

 mid-dorsal line of the head. Its skeletal support (fig. 17) 

 and nervous supply (fig. 27.) prove conclusively, (1) that 

 morphologically it does not belong to the head at all, and 

 (2) that it has secondarily passed forwards over the 

 cranium from behind. Further, an examination of the 

 connection between the dorsal fin skeleton and the skull 

 (fig. 17) shows us that the fin extends forwards in a 

 straight line over the cranium without being affected in 

 any way by the torsion of the head. (Cp. the course of the 

 fin indicated in fig. 1.) It is therefore certain that the 

 forward extension of the fin took place after the torsion 

 was complete. Hence it does not occupy the median line, 

 but follows what Traquair calls a " pseudomesial " course, 

 and, being a purely secondary character, may be 

 eliminated from the discussion. 



The second difficulty is the mischievous assumption 

 that the left eye has travelled over the top of the head to 

 the right side. The Jact is that the left eye is not on the 

 right side at all. Its presence there is purely illusory. 

 What has happened is that the whole of the 

 * Trans. Linn. Soc, vol. xxv., p. 263, 1865. 



