494 SCIENCE PROGRESS. 



addition to the dorsal fin in the middle of the back. It is 

 surprising- that the true structure of the tail in these animals, 

 discovered by Fraas of Stuttgart in 1892, is not described 

 in that generally excellent work the Royal Natural History, 

 where the obsolete view that the tail was horizontal like 

 that of Cetacea is repeated, 



Schultze points out that in the heterocercal tail the more 

 expanded lobe is the more flexible, while the continuation 

 of the vertebral column is much more rigid. Consequently 

 when the tail is moved horizontally the flexible lobe will 

 yield to the resistance of the water, and the surface of the 

 whole tail will be inclined to the horizontal plane, and will 

 move not horizontally but towards the more rigid edge, 

 that is in the case of sharks or sturgeon, upwards. Every- 

 one knows that an oar inclined under water and moved 

 horizontally will rise to the surface or go deeper into the 

 water according to the direction of the inclination, and the 

 same thing happens when a feather in which the vane is 

 broader on one side of the axis than the other is moved 

 under water. The greater flexibility of one sides causes the 

 feather or heterocercal tail to take an inclined position and 

 therefore always to move away from the horizontal plane 

 towards the more rigid edge. Consequently the tail of the 

 fish in which the upper lobe is more rigid rises when it 

 moves. As the fish is at the same time propelled forwards, 

 the effect of this would be to ram its snout into the ground, 

 and to counteract this it only needs to raise the edge of its 

 pectoral fins, the resistance of the water will then raise the 

 fore part of the body and the whole fish rises from the 

 bottom. According to Schultze this is the object of the 

 arrangement ; the fish being heavier than the water requires 

 a mechanism to enable it to swim easily from the ground. 

 The Ichthyosaur on the other hand was specifically lighter 

 than the water on account of its lungs and blubber, and 

 therefore needed a motor mechanism to counteract its 

 tendency to rise. Its tail when in motion sinks, and it only 

 needed to lower the front edge of its pectorals, to descend to 

 greater depths when it pleased. 



The relations of the fins in the flying fishes (Exocceti), 



