FLYING-DRAGONS. 169 



wrist, a fold of membrane, like the large fold 

 which served the purposes of flight. This mem- 

 brane was probably bounded in front by a strong 

 tendon which, before it reached the wrist, passed 

 into, or was attached to, a long, slender bone 

 known as the "pteroid." This bone has been 

 the cause of much speculation, and by some 

 authorities is regarded as answering to the 

 thumb. Probably, however, it corresponds to a 

 similar but smaller bone found in a precisely 

 similar position in the bird, and known as the 

 os prominens. The development of bones in 

 tendons subjected to considerable strain is a 

 common feature. When nodular in form they 

 are known as sesamoids. 



The wrist bones, as in the bird, were consider- 

 ably reduced in number. In the earlier ptero- 

 dactyles there were two distinct rows, made 

 up of several distinct bones ; but in the later 

 Cretaceous types, these became merged one into 

 another, so that those of each of the two origin- 

 ally distinct rows became fused, each row into a 

 single bone. 



The bones of the middle-hand, or metacarpus, 

 are seen also to undergo modifications. In some 

 species they were very short, whilst in others, as 

 in the species of the genera Cynorhamphus and 

 Ptenodmcon, they were extremely long, thus 

 giving an extra joint to the wing, comparable to 

 that formed by the elongation of certain ankle- 

 bones in the Frog, for example. What purpose 

 this elongation may have served is unknown ; 

 but it is interesting to note that those species in 

 which those middle-hand bones were elongated 



