EXTINCT ORDERS OF REPTILES. 225 



erect posture. The vertebral column is divisible into its 

 usual regions, the cervical region being long, and composed of 

 large and strong vertebrae, which do not carry ribs. In the 

 procoelous form of the dorsal vertebrae, the Pterosaurs agree 

 with many Eeptiles, and differ from all known Birds. The 

 sacrum consists of from three to six vertebrse, and the tail is 

 very short in Pterodactylus, and very long in Dimor-phodon. 

 It is, however, in the structure of the limbs and their sup- 

 porting arches that the Pterosaurs exhibit some of their most 

 remarkable characters. The pectoral arch is in many re- 

 spects very ornithic in its character, with long and slender 

 scapulae and distinct coracoids, but no clavicles are known. 

 The sternum (fig. 573, b) is of large size and broad form, 

 and resembles that of Birds in having a median ridge or 

 keel. Ossified costal cartilages or " sternal ribs " connect the 

 breast-bone with the ribs, as in Birds ; but there is the Eep- 

 tilian character that splint-like abdominal ribs are present. 

 The brachium and antibrachium present no peculiarities of 

 special note. The hand consists of four digits, of which, in 

 accordance with the views generally entertained, the inner- 

 most or thumb consists of two phalanges, the second or index 

 has three phalanges, and the third or middle finger has four 

 phalanges. All these three digits, also, are clawed. The 

 fourth digit or ring-finger, on the other hand, is greatly elon- 

 gated, and is clawless (figs. 573, 574). It carries the flying 

 membrane, and consists usually of four phalanges, though it 

 has only two phalanges in Ornithopterus, It should be men- 

 tioned that this elongated digit is sometimes regarded as 

 being really tlie index. 



As regards the hind-limb, the pelvis is very small ; the 

 ilia are produced both in front of and behind the acetabu- 

 lum, as in Birds ; and the foot generally consists of five 

 digits, of wliich four have sharp claws, while the " little 

 toe " may either be rudimentary or longer than the other 

 toes, serving in the latter case to assist in the expansion of 

 the flying - membrane (fig. 574, p). According to Prof 

 Seeley, the lower end of the tibia is really formed by an- 

 chylosis of the true tibia with the proximal tarsal bone, as 

 in Birds. 



VOL. II. P 



