388 
Mr. R. E. Tomes on White’s Thrush . 
(whereas the wings of the two species are very nearly equal), the 
shallowness of its keel, and its greater solidity. Its hinder margin 
is not more rounded than that of the Song-thrush or Redwing, and 
it is much less broken by incisions, these being of comparatively 
small size, and almost closed posteriorly. It would not be difficult 
to regard this species as one of a high degree of development 
amongst the Turdidat , but adapted to some special mode of life. 
Of the sternum of two other species kindly forwarded by 
Mr. Newton, I refrain to speak. One of them, Ixos obscurus, is 
but remotely related to the genus Turdus; and the other, 
Ixos xanthopygius , I should also suppose, from the inspection of 
the sternum, to be of equally distant relationship. I have not 
examined a specimen of the latter species. 
The modification in the form of the sternum of the species 
mentioned in this paper may be formally expressed in the fol¬ 
lowing manner- 
* Outward form typical. 
1. Sternum with the hinder margin nearly straight; lateral 
incisions nearly or quite closed posteriorly. 
Ex. Turdus vis civ or us, T. fuscatus, Monticola cyanea. 
2. Sternum with the hinder margin moderately straight; lateral 
notches rather deeper, not closed posteriorly. 
A. Organs of flight adapted for migratory habits. 
Ex. Turdus iliacus, T. pilaris , T. torquatus , T. migratorius, 
T. obscurus. 
B. Organs of flight not adapted for migration. 
Ex. Turdus musicus. 
** Outward form subtypical. 
1. Sternum with the hinder margin rounded, deeply notched, 
and exhibiting a degree of development in the adult similar 
to that of the immature of the foregoing and more typical 
group. 
A. Organs of flight adapted for migratory habits c 
Ex. Oreocincla aurea. 
B. Organs of flight not adapted for migration. 
Ex. Turdus merula. 
