HALGAN'S SPINE-TAILED RAY. 95 



in the isle of "Waigou, and also in New Ireland. 

 They furnished food for a great portion of the ex- 

 pedition during their stay at these islands. 



Almost every one is aware that a boat may be* 

 with certainty, urged forwards by what is called 

 sculling ; that is to say, by means of one oar passed 

 over its stern, and continually mov in the water 

 from side to side. Now it is precisely upon this 

 principle that the tail of fishes, moving from side 

 to side, operates in propelling them forward. It is 

 evident that the oar, on the one hand, and the tail, 

 on the other, in this alternate lateral motion, is 

 continually displacing a quantity of water great in 

 proportion to the length of the instrument em- 

 ployed, and consequently to the sweep which it 

 makes in its oscillations ; and it is by the resistance 

 which the water makes to this displacement by the 

 oar or tail, in coming from its extreme sweep to 

 the axis or mesial plane of the boat or fish, that 

 either is urged onwards. 



It will easily be understood why Nature has been 

 so solicitous to remove from the portion of the spinal 

 column, by which the tail of fishes is moved, every 

 possible cause of obstruction to its free lateral 

 motion why it is not burthened by the same kind 

 of ribs which are connected with the anterior por- 

 tion of the spine why all the viscera are placed so 

 far forward and why, lastly, there is either no 

 pelvis at all, or, at any rate, only a rudimentary 

 one, and, in general, unconnected with this part of 

 the body. The movements of the tail are only, or 



