REPORT ON THE ANATOMY OF THE PETRELS. 53 
third however being slightly the longer. There is a small foramen between it and 
the fourth. 
In Puffinus there is a distinct osseous bridge, developed on the anterior and distal 
surface of this bone, external to the impression for the tibialis anticus, which I have 
also seen present (on one side only) in Diomedea exulans. Usually the bridge remains 
tendinous. 
The three anterior digits are strong and well developed, the third and fourth being 
nearly equal in length. They have the normal number of phalanges, of which the 
basal one is always much the longest. In the Oceanitide the phalanx of the middle 
digit always exceeds the two succeeding ones, taken together, in length, whereas in the 
Procellariidz it is always shorter, considerably, than these two. 
The hallux is altogether absent in Pelecanoides, and is present only in the most 
rudimentary form, as already described (supra, p. 13) in the Diomedeine. In the 
Oceanitidee and remaining Proceliariine it is always present, though small, but is 
peculiar in consisting of only a single phalanx, which bears the claw (vide Pl. VI. fig. 14). 
It articulates, proximally, with a small metatarsal, which lies in its usual relationship to 
the cannon-bone formed by the conjomed metatarsals. 
In the ordinary Petrels the only pneumatic hones of the skeleton are the skull, lower 
jaw (around its angle), sternum (very slightly), and the cervical, dorsal, and some of 
the more anterior sacral vertebree. The limb bones are all filled with marrow. In the 
smaller forms indeed of both families only the skull, lower jaw, and a few of the most 
posterior cervical vertebrae seem to be pneumatic. As a rule there seems to be a gradual 
increase in the amout of pneumaticity of the bones correlated with the increase of size in 
the bird generally. 
In the Albatrosses the whole of the axial skeleton (excepting some of the ribs, 
the scapula, furcula, caudal vertebrae, and uncinate processes) becomes extensively pneu- 
matic, the sternum being especially so. The humerus, moreover, becomes hollowed and 
filled by air, which enters through the pneumatic foramina developed at the bottom of the 
infra-capitular fossa. 
The proportion of, the hind, as compared with the fore, extremity, as well as those 
between different segments of those limbs, are very different, as may be seen from the 
appended table of measurements (in millimetres), in the Oceanitide and the Procellariidee 
respectively. In the former the leg, as measured by the combined lengths of the femur, 
tibia, and metatarsus, and therefore excluding the toes, is longer than the wing (humerus 
+ ulna + manus (omitting the carpals). The tarsus is longer than the mid-toe or ulna, 
and at least twice as long as the femur. ‘The tibia is at least twice as long as the 
humerus, and much longer than the manus. 
In the Procellariide (including the Diomedeinz and Pelecanoides) the leg, measured 
in the same way, is shorter than the wing. The tarsus is not longer than the mid-toe 
