No: 1.] NORTH AMERICAN PASSERES. gI 
a correct diagnosis in differentiating the two series were they 
mixed up together. Perhaps, too, comparatively speaking, the 
cranial capacity in such a species as Szalia mexicana is greater 
than it is in Hesperocichla, and the day is not far distant when 
relative size of brain is to be given its weight in determining 
the position of a bird in the system. Passing to the remainder 
of the axzal skeleton in Hesperocichla, we find that although it 
is essentially and fundamentally the same as it is in a Bluebird, 
yet in its general facies it more nearly approaches the corre- 
sponding parts of the skeleton in Merula. This is quite true of 
the pelvis and sternum ; but at the best the differences among 
all three are but very slight,‘and a pelvis of a specimen of Szadca 
arctica in my hand is practically but the pelvis of Merula m. 
propinqua reduced, which I hold in the other, — character for 
character, —though I believe it is possible there may be one 
less vertebra in the pelvic sacrum in the Sza/za than there is in 
the Robin ; but I will not positively vouch for this until I have 
had the opportunity of counting them in a number of the zest- 
lings of the two species. 
To all intents and purposes, Werula migratoria has a skull 
and associate structures like those parts in a great overgrown 
Bluebird, so much are they alike. A good distinguishing feature 
here, however, are the osseous tympanic bullz in the Robin, a 
species wherein they are strikingly conspicuous and large. The 
truth of this may best be demonstrated by taking an anterior 
view of the skull of JZerula, where these bony, flaring ear- 
couches strike our eye at once, while in Hesperocichla and 
Szalia they would hardly attract any special notice. There 
would be no difficulty in deciding between the skeletons of 
Merula and Hesperocichla as to the species they belonged to, — 
a glance at figures 2 and 3 will satisfy us as to the truth of this, 
where I have drawn the superior aspects of the skulls of the 
representatives of these two genera, and similar differences may 
be detected in the remainder of their skeletons. 
Turdus a. pallasit as representing its genus has a skeleton 
the very counterpart of the skeleton in Hesperocichla, being in 
size about one-third less. 
Thus it will be seen how, osteologically, our American 7ur- 
din@ are closely linked together, and they naturally form a very 
well-defined sub-family of Birds, but I fail to discover anything 
