148 On the Tarsus and Carjms of Bi7'ds. 



Prof. Wyman has generously placed at my disposal the 

 tarsus of au embryo heron, with other specimens of the 

 same bird, and authorized me to embody their peculiar 

 features in this paper. Accompanying the specimens, the 

 following letter was received, which I have the liberty of 

 publishing : 



Dear Prof. Morse, — 



111 a receut examination of the bones of the leg of some unfledged 

 herons (supposed to be Ardea coerulea) with refei'ence to their ossifica- 

 tion, to which I was led by your admirable embryological studies of the 

 limbs of birds, I found a bone accompanying the groove on the front of 

 the lower end of the tibia, which does not agree with any description of 

 these parts I liave seen. It is a style-shaped bone, ends in a sharp point 

 above, has the lower end, which is on a level with the lower end of the 

 tibia, blunt and rounded, and almost exactly resembles, in the older speci- 

 mens, the fibula of the same leg inverted. 



It is about one-fifth as long as the tibia, and as appears from several 

 specimens of diffei'ent ages, grows, for a time at least, in proportion to 

 the other parts. 



In the older specimens two nodules of bone are seen, side by side, in 

 the cartilage part below the tibia, one corresponding to each cond.yle, as 

 you have pointed out in other birds. These belong to the near portion of 

 the tarsus, and may therefore be supposed to represent the astragalus 

 and calcaneum. Thus we have these two bones entirely apart from the 

 pretibial bone described above. 



As regards the homology of this last named piece, the most natural 

 supposition is that it is the ascending process of the astragalus to which 

 attention has of late been called by Huxley and Cope, in discussing the 

 aftuiities of birds and Dinosaurian reptiles. 



Its mode of development, however, leads to the belief that it is not, 

 properly speaking, a process of either of the tai'sal bones, but a distinct 

 bone, for it not only has an independent ossification, but is alreadj' far 

 advanced in this process, before the ossification of either of the tarsal 

 bones is begun. It has occurred to me that the part in question might 

 have been originally the lower portion of the fibula, from which it had 

 become detached by absorption, but it has not at any time been observed 

 to be continuous with this bone, and further, it continues to grow from 

 below upward, as the young bird gets older, instead of becoming shorter 

 and shorter, as it ought, if this supposition was true. 



Observations are now wanted to show whether the ascending process 

 of the astragalus, as seen in the ostrich and other birds, is really an out- 

 growth from, and therefore a process of, one of the tarsal bones, or 

 whether it ossifies independently, and subsequently becomes united with 

 it. Should this last supposition be decided in the afllrmative, then we 



