the Structure of Hierococcyx b^c. 601 
of them entirely disappear. -Thus in Hierococcyx, in 
Cuculus, and in the allied genus Cacomantis the ventral tract 
of either side ends in a single row of feathers through the 
suppression of the two rows which are found in addition 
higher up in the tract. 
The second feature which I believe to be of importance for 
purposes of classification is that afforded by the muscles of 
the thigh. Garrod had already on these grounds divided a 
Cuculine from a Centropine series ^. And in my memoir 
upon the classification of this order or family I adopted the 
facts given by Garrod (adding somewhat to them) in my 
definitions of the three groups into wdiich I proposed — and 
indeed still propose — to divide the existing Cuckoos. Id 
Centropus, &:c., the muscle-formula of the leg is the full 
formula A B X Y with the ambiens. In Cuculus and its 
allies the muscle-formula is reduced by the loss of B, the 
accessory femoro-caudal muscle. It will be observed that 
the loss of this muscle is coincident with the loss of the outer 
band of the ventral feather-tracts. One cannot help thinking 
that the apparent loss is a real loss in both cases, a reduction 
from a more complicated state of affairs. In any case there 
is no Cuckoo known with the formula A X Y which possesses 
the outer band of the ventral feather-tract ; and, conversely, 
no Cuckoo known which possesses that outer band that has 
not also got the full muscular formula A B X Y. 
Hierococcyx proves to be no exception to this rule, at 
present universal. The muscle-formula of the thigh is 
A X Y with, of course, the ambiens muscle also. 
The third structural feature upon which I based my 
attempted classification of the Cuculidse concerns the form 
of the syrinx. This organ is developed along two lines in 
this group of birds. In many forms the syrinx is apparently 
of the most typical avian form, in which tlie intrinsic muscles . 
of the syrinx are attached to a bronchial semi-ring close to the 
point at which the trachea divides into the tw^o bronchi. 
This form of syrinx is generally known as the tracheo- 
bronchial, and is — as is well known — the most usual form 
* Coll. Scientific Papers, ed. by W. A. Forbes, London, 1881, p. 220. 
