xv. 1 8 



PTERODACTYLS 



427 



limb bones became light, the skull bones fused, and the jaws toothless 

 and beaked. This parallelism in lines known to be distinct, although 

 of remote common origin, is similar to that which we have noticed 

 before in aquatic animals, and it can be interpreted as showing that 



f¥. 





Fig. 248. The skeleton of a pterodactyl. 



A, extra wrist bone; C, coracoid; D, elongated digit; F, femur; FF, fin; H, humerus; 

 MC, metacarpal; P, pelvis; RU, radio-ulna; SC. scapula; ST, sternum; 7', tail; TF. 

 tibio-fibula; lowing. (From Thompson, The Biology of Birds, Sidg\vick& Jackson, Ltd.) 



populations with similar genotypes will respond to similar environ- 

 mental stimuli in the same way. 



The pterodactyls are most commonly found in the Jurassic strata, 

 less often in the Cretaceous. Many specimens have been found in 

 marine deposits and seem to have been fish-eaters. The characteristic 

 features that have produced the pterodactyl structure from a thecodont 

 ancestry may be described as a lengthening of the head and neck, 

 shortening of the body and ultimately of the tail, lengthening of the 

 arms and especially of the fourth digit, shortening of the legs, and 

 development of the ventral parts of the limb girdles. These are the 



