388 Mr. R. F. Tomes on White's Thrush. 



(whereas the wings of the two species arc 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 Turdida, 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 Twdus; 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. Twdus viscivorus, 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. Tardus iliacus, T. pilaris, T. torquatus, T, migratorius, 

 T. obscurus. 



B. Oi'gans 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. 

 Ex. Oreocincla aurea. 



B. Organs of flight not adapted for migration. 

 Ex. Turdus merula. 



