454 BRITISH FOSSIL REPTILES. 



Both proximal and distal surfaces show well-defined, concave articulations. 

 On the more concave surface (fig. 5) there is an oblong, articular depression (</), 

 continuous at the margin (/') with a surface on the opposite side of the bone ; 

 a more irregular undulated channel, deepest at the middle part (i), occupies the 

 rest of the surface, but the end of the bone opposite (^) has been broken away. 

 Fig. G shows two shallow, articular channels (A- and l), partly divided near the end 

 (h) by a tract of non-articular surface. 



In birds the base of the metacarpal of the digitus medius has the " os 

 magnum " connate therewith, it also becomes confluent with the bases of the 

 second and fourth metacarpals. Between this compound bone and the anti- 

 brachium two distinct carpal bones partially intervene, being wedged between the 

 metacarpus and antibrachium, one on each side. The Pterodactyle, in the com- 

 plete separation of the metacarpus from the antibrachium, by two successive 

 carpals, answering to the two rows, adheres more closely to the Reptilian type ; 

 but difi'ers in the much greater expanse and complexity of the carpals, and in 

 their minor length. 



Ungual Phalanx (PI. 14, figs. 11 and 12). 



The ungual phalanx {Pterosauria, PI. 14, figs. 11 and 12), accords in size with 

 that of the limb indicated by the carpal bones (figs. 5 — 10). The articular surface 

 presents two trochlear concavities, extended vertically, narrow transversely, divided 

 by a median ridge ; the upper angle is rather produced ; below the trochlea is a 

 small depression, and below this the bone projects in the form of the rough pro- 

 tuberance for the flexor tendon. On each side of the phalanx is the curved 

 vascular groove, beneath which, in some specimens, the bone slightly expands. 

 In one specimen a second, more shallow groove is shown on one side, nearer the 

 upper margin of the bone. 



