9 
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 fact 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. xxvy., p. 263, 1865, 
