THE TAIL 



slope of their contour and nice variation in thickness, as the situations of 

 the various stresses dictate, are very suggestive of their evident fitness 

 for function. The epidermis is, of course, very thin as it is over the re- 

 mainder of the body, and beneath this is a pure white, fibrous tissue, 

 somewhat fatty and very elastic in texture, which has phenomenal tough- 

 ness and yet may be shced with a knife without difficulty. The strength 

 of this tissue is forcibly impressed upon an observer as he watches a 

 whale of 75 tons being drawn up the slip tail first. If the flukes catch 

 beneath some obstruction either the latter is torn rrom its moorings or 

 the flukes snap into place with an abruptness and strength that shakes 

 the entire animal, but very seldom is any serious damage suflfered by the 



_ .^^ X 'rmsismS^^^r- 



Figure 33. Suggested restoration of caudal outlines, illustrating progressive de- 

 velopment, of the Ichthyosauridae: (a) Mixosaurus ; (b) young and (c) 

 adult stages of Stenopterygius ; and {d) Ichthyosaurus (after Fraas). 



flukes themselves. It is difficult to understand why, during the time 

 that it took for cetacean flukes to reach their present stage of perfection, 

 the caudal vertebrae themselves did not experience profound alterations 

 in the way of broadening, and as a result we must believe that any un- 

 yielding stiffener of such a sort would not prove advantageous. Of 

 course there may ultimately be acquired within the caudal lobes an equip- 

 ment of supporting cartilages, in which ossification centers might ap- 

 pear, but as there is as yet no sign of any such development there is no 

 reason to think that there ever will be. 



In all the Cetacea the transition from peduncle to flukes is very 

 abrupt, the latter always diverging at almost a right angle. The cross 



[203] 



