70 WATER REPTILES OF THE PAST AND PRESENT 



among turtles there never has been an actual increase in the num- 

 ber of vertebrae; so also in the elongation of the digits the normal 

 number of three in each digit has never been exceeded, except among 

 the river turtles, where there are four in the fourth digit — possibly 

 a relic of original conditions rather than the beginning of hyper- 

 phalangy; but the individual bones have become greatly elongated. 

 In living reptiles, birds and mammals of the land, the fifth toe is 

 always shorter than the fourth. In the seals, the sea-otter, and to 

 a less degree in the muskrat, the fifth toe has become elongated. 

 And the elongation of this toe is the first and most decisive indica- 

 tion of a webbed foot of strong propelling power among the aquatic 

 reptiles of the past, as exemplified especially by the proganosaurs. 

 Finally, in one order of extinct reptiles, the ichthyosaurs, there has 

 been an actual increase in the number of digits, in some to as many 

 as nine in each paddle. 



In addition to all these modifications of the skeleton, the bones 

 themselves tend to become softer and more spongy in aquatic 

 animals. The bones of the whale, as is well known, are very spongy 

 in texture, and those of the seals and sea-Hons contain an unusually 

 large amount of oily matter. So, too, the bones of the extinct water 

 reptiles — of many of them at least — were more spongy than those 

 of their land relatives; and this is due in part perhaps to their 

 lessened use as muscular supports, in part perhaps to the necessity 

 of a lessened specific gravity. As a rule sea-animals need to be of 

 the same specific gravity as the water in which they live, or a little 

 less. The bones of the living sirenians, the manatees and dugongs. 

 so far from being hght and porous, are unusually dense and solid. 

 The sirenians live habitually at the bottom of shallow waters, feed- 

 ing upon vegetable growths; and doubtless their bottom-feeding 

 habits account for the soHdity of the bones. A whale would float 

 to the top. while a dugong would sink to the bottom, on the relaxa- 

 tion of all muscular movement. And we shall see that certain 

 reptiles in the past had in all probability like bottom-feeding habits, 

 because of the solidity of the bones of their skeletons. 



Many birds and fishes have a pecuHar ossification of the usually 

 tendinous outer covering of the eyeball, called the sclerotic mem- 

 brane. These ossifications fomi a flattened or somewhat pro- 



