84 SHUFELDT. (Vou. Ill. 
form, the Finch form, or the Crow form, is pushed to the most 
_ extreme point of differentiation ; here we shall find that between 
the two outposts thus established there exists a regular chain of 
intermediate stations, so intimately connected that no precise 
lines of demarcation can be drawn cutting off one from the 
other. 7 
Now these observations apply nowhere more cogently than 
they do in the study of the osteology of the Passeres, and it is 
positively almost beyond the range of possibilities, sometimes, 
to distinguish between the skeletons of certain closely related 
species, while in other instances very excellent differentiating 
characters may be found; and these latter will be brought con- 
spicuously into the foreground and made the most of, when 
met with. It matters but little which family we select to com- 
mence this series of osteological comparisons of this order of 
birds, though for convenience’ sake, solely, the writer proposes 
to take the genus Szala of the family Zurdzde@ first in order, 
because on this occasion it is desirable to follow the sequence 
adopted by the A. O. U. Check-List. Not that we are at all con- 
vinced that the Zurdide constitute the most highly specialized 
group of birds in our avifauna, for we have decided opinions and 
leanings quite the converse to any such an hypothesis ; but more 
for the reason that we find in such a skeleton as is furnished us 
by Szalia a type, as it were, of what we have been given to 
understand the passerine skeleton, through the numerous de- 
scriptions of a host of former anatomists, purports to be. 
We have divided our Yurdide into two sub-families — the 
Myadestine and the Turding ; the first containing the genus 
Myadestes, and represented by the sole species JZ. townsendiz, 
and the second the genera 7urdus, Merula, Hesperocichla, Cyane- 
cula, Saxicola, and Szala,—each containing one or more well- 
known species of Thrushes or Thrush-like birds. 
There are four species of Bluebirds in the genus Sza/za, and I 
have compared the skeletons of all. It is hardly necessary to 
say that in these several species the skeleton is essentially the 
same, although one might, after long and careful study and com- 
parison, be enabled to pick out those belonging to the different 
varieties. In the most typical species, each seems to possess 
1 NEWTON, ALFRED, F.R.S., etc., etc., Article “ Ornithology,” Brit. Encycl., oth 
ed., Vol. XVIII, p. 47. 1885. 
