240 THE JOURNAT. OF GEOLOGY. 
achians the vertebral column ends in the upper, in Ichthyosaurus in 
the lower lobe of the caudal fin. An explanation of this fact has lately 
been given by Professor Franz Hilhard Schulze’ of Berlin. _ 
By the lateral motion of a caudal fin like that of the Selachians in 
form, the auimal can not only be propelled forwards, but also upwards 
or downwards. The motion will be forwards and upwards if, as in the 
sharks, the upper edge of the caudal fin is the stronger one; it will be 
forwards and downwards if, as in the Ichthyosauria, the lower edge of 
the fin is the stronger, supported by the caudal vertebrae. The sharks, 
as it is well known, are heavier than the water; the motion upwards, 
therefore, produced by the motion of the lower portion of the tail, is of 
the highest value for the sharks. The Ichthyosaurs on the other hand 
were as reptiles with lungs, and with the extensive layer of fat, lighter 
than water. For these the motion of the upper portion of its caudal 
fin, producing a downward motion, is of the greatest value. 
G. Baur. 
* SCHULZE, FRANZ EILHARD: Uber die Abwartsbiegung des Schwanztheils der 
Wirbelsaule bei Ichthyosauren (Sitzungsber. Berliner Akad. 1894, p. 1133). 
