132 REPTILES 



lations are only essential to climbing snake-like reptiles. To 

 the second part of the question no answer seems forthcoming. 

 This, however, is not all, for we must either regard iguanas as 

 the direct ancestors of snakes, sea-serpents, and dolichosaurians, 

 or we must consider that zygosphenes were present in the an- 

 cestral stock of all the Squamata,'or, finally, that they must 

 have been developed independently in each of the groups in 

 which they occur. If the latter be the solution of the puzzle, it 

 is marvellous that these structures should be practically identical 

 in all the groups. 



Passing on to modifications in the skeleton of the limbs, we 

 have to notice the fore-limbs, or " wings," of pterodactyles. In 

 these there is nothing remarkable, so far as length is concerned, 

 in either the first (humerus) or second (radius and ulna) seg- 

 ments ; the metacarpus is, however, so lengthened as to be 

 equal in this respect to the radius and ulna, but the real carrier 

 of the wing is the enormously elongated fifth or outer digit (the 

 first being lacking), each of the first three joints of which is 

 considerably longer than the radius. In the mode of construc- 

 tion of the wing, pterodactyles therefore present no sort of 

 parallelism to birds. It may be added that in some of the 

 earlier forms (Dimorphodon) the proportionate length of the 

 metacarpus is much less than that mentioned above. 



The second noteworthy modification of the fore-limb occurs 

 in the iguanodon, in which the first digit, or thumb, has assumed 

 the form of a short and thick conical spine, which was probably 

 sheathed with horn during life and may have been employed in 

 fighting. 



Turning to marine forms, in which the limbs were modified in- 

 to flippers or paddles, we find the simplest skeletal type of this 

 kind in the turtles, in which the bones of the digits are abnor- 

 mally lengthened, without any increase in number, or marked 

 alteration in their form. Moreover, the bones of the upper 

 segments of the limb (humerus and radius and ulna) are not 

 abnormally shortened. Much the same may be said with regard 

 to the extinct pelagic crocodiles. When, however, we come to 

 the extinct sea-serpents, or Pythonomorpha, we find that while 

 the digits are so normal, the aforesaid three bones of the upper 

 segments have become shortened and widened, that their lengths 

 scarcely exceed that of the first row of bones in the digits. 



