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 Oceanitidas the phalanx of the middle 

 digit always exceeds the two succeeding ones, taken together, in length, whereas in the 

 Procellariidse 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 Diomcdeina;. In the 

 Oceanitidas and remaining Procellariinse it is always present, though small, but is 

 peculiar in consisting of only a single phalanx, which bears the claw (vide PI. VI. fig. 14). 

 It articulates, proximally, with a small metatarsal, wdiich lies in its usual relationship to 

 the cannon-bone formed by the conjoined metatarsals. 



In the ordinary Petrels the only pneumatic bones 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 vertebras. 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 vertebras 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 vertebras, and uncinate processes) becomes extensive!)* 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 Oceanitidas and the Procellariidas 

 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 Ions; as the femur. The tibia is at least twice as lona; as the 

 humerus, and much longer than the manus. 



In the Procellariidas (including the Diomedeinas and Pelecanoides) the leg, measured 

 in the same way, is shorter than the wing. The tarsus is not longer than the mid-toe 



