in the Classification of Birds. 15 
the organization of any particular bird-form presents us 
with, and by no means are we to rely upon them alone, or 
even when afew other sets of structural characters seem to 
indicate a bird’s affinity. 
To illustrate my point, let us turn for a moment to the 
Swifts and Humming Birds; here we have two groups 
which for years past have been associated together as 
allied forms by systematists, and Garrod, too, seemed to 
believe in their affinity. Why? Because the formula for 
the thigh muscles in each case was found to be a; the 
sternum had in each case an unnotched posterior border; 
and neither swifts nor hummers possess intestinal coeca. 
Yes, this all may be so, but a// the rest of the organization 
of these birds is as widely different as one can well 
imagine, and consequently they belong to very different 
orders of birds. This latter statement gains weight when 
we come to think that aside from the formula for the 
thigh muscles being the same in Cypseli and Trochili, 
their pelvic limbs otherwise are by no means alike in other 
particulars; and the sternum is, too,.of a very different 
pattern in each case, although, as I say, each possesses an 
entire posterior xiphoidal margin. 
My object will have been attained here when I have 
presented you with a brief description of each of these 
muscles, and directed your attention to them in my draw- 
ings which illustrate this paper. As will be seen by the 
figures, I found them a// present in the pelvic limb of Geo- 
coccyx californianus, and this is. the bird I have chosen to 
illustrate my remarks in what follows. 
6. The ambiens (Fig. 9): This muscle arises from the 
apex of the prominent prepubic spine of the pelvis, and 
the fibres passing directly down to the inner side of the 
femur, and parallel with that bone, form a strong fusiform 
muscle. Asit approaches the patella it terminates in a 
small flattened tendon, which, piercing the fascial envelop 
of the knee-joint below the inferior apex of that sesamoid, 
passes round the joint to become finally lost to the outer 
side and opposite the summit of the tibia, where some of 
its tendinous fibres merge with the fibres of origin of the 
