152 On the Tarsus and Carpus of Birds. 



Figure 7 represents the tarsus of an embryo bird, with 

 the pretibial bone introduced, bearing the same relation that 

 it afterwards does to its own tarsals ; and figure 8 repre- 

 sents the pretibial bone and its actual relation to the tAVO 

 tarsals, as seen in Professor Wyman's oldest specimen, the 

 tibia now having widened so as to include the two tarsals 

 within its lateral boundaries, and consequently including 

 the pretibial bone also. That the tibia widens at its distal 

 extremity in that way, so as to equal in width not only the 

 two tarsals, l)ut the three metatarsals, may be seen by re- 

 ferring to plate IV ; and indeed to suppose that it would do 

 so is reasonable, since the excessive reduction of the fibula 

 naturally enhances the greater proportionate development of 

 the tibia ; and by this excess of growth, the pretibial bone, 

 or intermedium, finds its anomalous position in front of the 

 tibia. 



In connection with this elongated intermedium, it is inter- 

 esting to note that in certain lizards where the intermedium 

 is absent, the centixde takes on the elongated and slender 

 form, and is wedged between the other tarsals. 



If further investigation should prove the correctness of 

 this interpretation, wc have an interesting stage in the con- 

 ditions of this bone represented, namely, that in birds the 

 intermedium is at first a separate bone, as in the lower rep- 

 tiles, but finally it anchyloses with the astragalus, as in 

 higher vertebrates, thus proving the correctness of Gegen- 

 baur's statement that the astragalus of higher vertebrates 

 represents the tibiale and intermedium connate. 



Thus we must recognize in birds the presence of four 

 tarsal bones, and at least four carpal l)ones. 



Concluding Observations. — At an early stage bf the 

 embryo the leg and the wing are almost precisely alike, and 

 even after the principal bones have made their appearance 

 the two appendages are remarkably alike in the form and 

 proportion of their parts. This similarity was noticed by 



