142 ANIMAL LIFE PAST AND PRESENT. 



and that of a bird is a good instance of what compara- 

 tive anatomists term an adaptive resemblance ; that is, 

 a resemblance caused by the circumstance that a par- 

 ticular organ or bone has to subserve the same purpose 

 in two particular instances, and not as indicating any 

 direct relationship between the owners of such struc- 

 tures. It will further be observed from the figure 

 that the skeleton of the Pterodactyle differs from that 

 of an ordinary bird by the absence of the so-called 

 "merry-thought" or furcula. Since, however, that 

 bone is either rudimentary or absent in the flightless 

 birds allied to the ostrich, it cannot be regarded as a 

 feature of first rank in distinguishing Pterodactyles 

 from birds. 



Having thus shown that Flying Dragons cannot be 

 classed with birds, it remains to mention why they 

 should be placed with reptiles rather than with 

 mammals. This point is, however, at once decided by 

 the circumstance that the skull is jointed to the back- 

 bone by a single knob-like articulation, or condyle, 

 instead of by the two condyles found in all mammals 

 (see page 68). If further proof were wanting, it is 

 found in the circumstance that each half of the lower 

 jaw consists of several totally distinct bones, as in all 

 birds and reptiles ; whereas in mammals it is composed 

 of but a single piece. 



With this glance at the general features of the 

 skeleton of the Flying Dragons, we may proceed to 

 notice some of the peculiarities of the different kinds 

 of these creatures, and also what is known as to the 



