CRETACEOUS FORMATIONS. 19 



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

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

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

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

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

 Fig. 6 shows two shallow, articular channels (/c and i), 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 differs in the much greater expanse and complexity of the carpals, and in 

 their minor length. 



Ungual Phalanx (Tab. IV, figs. 11 and 12). 



The ungual phalanx (Tab. IV, 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. 



