134 REPTILES 



from reptiles with limbs of more normal structure. For instance, 

 in the genus Merriamia the humerus is longer and more 

 constricted in the middle than is the case in Ichthyosaurus ; 

 while the radius and ulna (as also the tibia and fibula in the 

 hind-limb) likewise retain the characters of "long bones," their 

 length markedly exceeding their width, while they are con- 

 stricted in the middle and separated from one another by a 

 space, so that they are essentially different in character from 

 the bones of the carpus by which they are succeeded inferiorly. 

 Again, the carpus bones themselves as well as those of the 

 digits are proportionately longer and less squeezed together 

 than in Ichthyosaurus. It will also be noticed that the external 

 margins of the bones of both the outer and inner digits (such 

 digits being only three in number) are notched. That such 

 notches represent the last vestiges of the shafts of " long bones " 

 can scarcely be doubted, despite the fact that they also occur 

 in the bones of the carpus (the two transverse rows immediately 

 below the radius and ulna) which are not normally " long 

 bones ". 



Beyond this stage it has not been found possible to carry 

 the derivation of the complex paddle of the Liassic ichthyosaurs, 

 but it may be taken for granted that if we had the full series 

 of forms, we should find a transition to reptiles with limbs 

 adapted for terrestrial movement. It may be added that in the 

 supreme culmination of the ichthyosaurian stock {Ophthalmo- 

 saurus) three bones articulated with the humerus in place of the 

 two characteristic of Ichthyosaurus. 



Finally, it is scarcely necessary to emphasise the fact that 

 all the various types of paddles referred to in the preceding 

 paragraphs have been evolved independently of one another 

 from terrestrial or semi-aquatic reptiles. 



In the majority of reptiles the nostrils open posteriorly 

 into the mouth near the front or middle of the palate, so 

 that the tube connecting the external, or true nostrils, with the 

 internal or posterior nostrils (choanal) is comparatively short. 

 In modern crocodiles and gharials, on the other hand, the 

 posterior nasal apertures have been carried back close to the 

 occipital region of the skull ; this adaptive arrangement being 

 brought about by the development of a secondary flooring to 

 the skull, or what would be called in human anatomy a back- 



