2 6 Sir Everard Home on the fossil bones of an animal whose 
As this form of the sternum appeared at the time quite 
new, I, was very anxious not to fall into an error, and was 
re-examining, with Mr. Clift, the different specimens, when 
it struck him that there was something similar to this mecha- 
nism in the sternum of the ornithorhynchus paradoxus : this 
remark led to a comparison of the bones, and they were 
found to have a general agreement that, could not have been 
expected. 
In the ornithorhynchus, the first bone of the sternum, at its 
upper end, has two lateral processes, which are connected 
with a similar process from each of the scapulas ; underneath 
the first bone are two flat bones united together, which union 
is covered by the first bone. On the outer edge of these flat 
bpnes, there is a broad process continued down from thq 
scapula ; in this process es the hollow to which the first bone, 
or os humeri of the pectoral fin is articulated. 
It will be seen that the difference between these bones of the 
sternum in the fossil skeleton and that of the ornithorhynchus, 
consists in the fossil skeleton having a clavicular bone, which is 
wanting in the other; and the ornithorhynchus having a long 
process from the scapula, in which is the cavity of the shoul- 
der joint, wanting in the fossil skeleton. These slight diffe- 
rences are not readily perceived in looking at the parts,, as 
will be seen on inspecting the annexed engravings, PI. II. 
Fig. 1, and 2. 
The mechanism which has been described,. gives a very 
unusual surface for the muscles attached to the sternum, 
which move the first bone of the pectoral fin ; and upon exa- 
mining them in the ornithorhynchus, I find there is not only 
