136 REPTILES 



jaws formed a horny beak devoid of teeth. This gives an 

 opportunity to direct attention to the fact that several groups 

 of reptiles have developed independently of one another horn- 

 sheathed beaks and discarded teeth. That such a development 

 is in some manner connected with food and feeding, may be 

 considered almost certain ; but it is difficult to say what is the 

 precise reason for the modification, seeing that while forms 

 with beaks, such as a large number of chelonians, are herbivor- 

 ous, the pterodactyles were carnivorous. The Chelonia are 

 the only ordinal group of reptiles of which all the members 

 have discarded teeth in favour of a horny beak ; and we are 

 still in the dark as to the kind of toothed reptiles from 

 which tortoises and turtles are descended. There are, how- 

 ever, several groups of which the later and more specialised 

 representatives have lost all or most of their teeth and acquired 

 a cutting beak, probably ensheathed during life in horn. 

 Among such reptiles are the dicynodonts, belonging to the 

 order Anomodontia, in which the males at any rate retained a 

 pair of large tusks in the upper jaw, but had the rest of the 

 jaws converted into a cutting beak. In certain other skulls, 

 which probably indicate the females of Dicynodon, although 

 they have been referred to a genus apart ( Udenodon), even the 

 single pair of tusks was lacking and the entire jaws were beak- 

 like. The pterodactyles present another instance where the 

 more specialised types (Pteranodon, etc.) have replaced teeth 

 by a horny beak ; and the same seems to have been the case 

 with the specialised ichthyosaurs of the genus OplitlialmosauruS) 

 in which, however, a few small functionless teeth may have 

 persisted. 



Reverting to the giant pterodactyles, it may be mentioned 

 that in point of size a species from the Cretaceous strata of 

 Kansas exceeded the largest albatross, the span of wing being 

 no less than 23 ft., while the skull is little short of 40 in. in 

 length. The latter dimension is, however, calculated to give 

 an exaggerated idea of the size of this "dragon," for the skull 

 is extended nearly as far behind the vertebral column as it is 

 in front. In fact, this part of the skeleton resembles a crutch- 

 stick or a pickaxe, the vertebral column forming the stick and 

 the skull the cross-bar. Apparently this backward extension 

 of the occipital region of the skull was intended as a counter- 



