NO. 5 TELESCOPING OF THE CETACEAN SKULL 37 



ceeded as though influenced by forces which were relatively simple 

 in both directions. In the baleen whales, on the contrary, the op- 

 posing forces may have been more complex. They perhaps might be 

 analyzed as follows: (i) From in front; (a) the resistance of the 

 water through which the animal moved, (b) the constant downward 

 pull exercised by the increased weight of the rostrum and jaws con- 

 sequent on their enlargement as the framework of the food-straining 

 apparatus, and (c) the very unusual downward pviU which must 

 occur when the animal in feeding swims forward with the lower 

 jaw lowered and the under part of the mouth distended by the 

 enormous quantities of water taken in for straining through the 

 baleen plates : (2) From behind ; (a) the forward push of swimming, 

 and (b) the upward pull needed to counteract ib and ic. The 

 presence of some special downward-pulling force seems to be re- 

 quired as part of the explanation of the great forward extension of 

 the upper margin of the occipital shield which has been one of the 

 noticeable features in the developmental history of the baleen whale 

 group. This oblique extension of the shield gives increased area for 

 the insertion of the muscles which hold the head in its horizontal 

 position, and at the same time it gives increased length to the power 

 arm of the lever by which any downward force applied to the anterior 

 part of the head is opposed. An analogous mechanical problem has 

 been solved by a parallel though much less extreme modification of 

 the skull in some of the burrowing rodents, particularly in those 

 which have not developed specially enlarged fore feet. Such animals 

 make free use of the teeth and snout in digging ; consequently the 

 fore part of the skull must be pressed upward against the soil through 

 which a passage is being made. This active upward-forcing of the 

 rostrum against a resistant medium appears to have brought into 

 play the same muscles that would be required for maintaining the 

 horizontal position of the fore-weighted head of a baleen whale. It is 

 therefore not surprising to find that a distinct parallelism should be 

 present between the vertical portion of the skulls of such widely 

 unrelated mammals as Rhachiancctes and Spalax. In the rodent 

 (pi. 8, fig. 8) the occipital shield has extended far enough forward 

 over the parietals to reduce their exposed portion to small elements, 

 while in the whale (pi. 8, fig. 3) it has obliterated the parietals on the 

 vertex and has continued forward beyond them far enough to reduce 

 the frontals in their turn to elements whose size relatively to the 

 neighboring structures is not greatly unlike the exposed area of the 



