THE VOYAGE OF THE * CHALLENGER.' 425 



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 

 Oceanitidae the phalanx of the middle digit always exceeds the two suc- 

 ceeding ones, taken together, in length, whereas in the Procellariidae 

 it is always shorter, considerably, than these two. 



The hallux is altogether absent in Pelecqno'ides, and is present only 

 in the most rudimentary form, as already described (supra, p. 377), in the 

 Diomedeinae. In the Oceanitidae and remaining Procellariinae it is always 

 present, though small, but is peculiar in consisting of only a single 

 phalanx, which bears the claw (vide PL XXII. fig. 6). It articulates, 

 proximally, with a small metatarsal, which 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 vertebrae. 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 amount 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 pneumatic, 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 Oceanitidse and the Procellariida? 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 -fmanus, 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 Procellariidae (including the Diomedeinee and Pelecano'ides) the 

 leg, measured in the same way, is shorter than the wing. The tarsus is 

 not longer than the mid-toe (except in Procellaria where it is just) but Zool 



shorter, and the same is always the case when it is compared to the ulna. Exp. vol. iv. 



pt. xi. p. 54. 



