52 THE VOYAGE OF H.M.S. CHALLENGER. 
There is not very much divergence from the general form of pelvis described here 
amongst the various forms of Petrels. In the Albatrosses it becomes very narrow, 
especially anteriorly, the renal fossee being correspondingly narrow and deep. The 
bones entering into its composition, and supporting it become extensively pneu- 
matic. Anteriorly the ila unite, or nearly so, over the neural crest, whilst pos- 
teriorly the foramina between the transverse processes of the sacral vertebrae become 
reduced to small holes, owing to the increased amount of ossification. A tend- 
ency to similar increased ossification in these parts is discernible in the larger forms 
of Petrels. 
In Pelecanoides the ischia are nearly straight along their posterior margin. 
In Cymochorea and Procellaria, as well as in the Oceanitide, the pelvis generally is 
weaker, with its posterior region more deflexed. But that of the two former genera may 
be distinguished from that of the Oceanitidee by the obsolete condition of the posterior 
iliac spine, the weakness of the post-acetabular ridge, and the smaller size of the ilio-sciatic 
and obturator foramina. 
Posterior Extrenity.—The fen is short, and more or less curved, most so in 
Pufjinus. The head is a little elevated, and set on nearly at right angles to the long 
axis of the bone. ‘The external condyle descends lower than the internal, and has a 
well-marked posterior trochlear surface. The femur of the Oceanitidze is a stronger and 
better developed bone, particularly at the two extremities, than it is in the Procellariidee 
of corresponding size. The tibia is long, especially in the Oceanitidee, where it is at 
least twice as long as the humerus. It has a very large and strong cnemial process, 
best developed in the genus Puffinus, for the attachment of muscles, rising high above 
the articular faces of the femur (vide Pl. VI. fig. 13). Its distal end is more or less 
antero-posteriorly compressed, and has an osseous bridge for the extensor communis 
digitorum. The ridge for the fibula extends for about one-quarter the total length of 
the shaft of the bone, beginning a little below the external condylar facet. 
The fibula is a delicate, styliform bone, which may be two-thirds as long as the 
shaft of the tibia. 
The tarso-metatarsus has a smooth, rounded, interarticular prominence proximally, 
and a calcaneal eminence, with two deep grooves, which may become converted into 
canals, for the passage of the flexor tendons. In the Diomedeinz this calcaneal process 
is feebly developed, with but a single groove internally, and a broad trochlear surface, 
with two shallow furrows, externally. The antero-external margin of the bone is sharply 
keeled. In such forms as Puffinus this keel becomes very sharp and prominent, owing 
to the 
erooved. 
Of the articular trochlee at its distal end, the inner is the shortest and most 
oblique. Those for the third and fourth digits are more nearly equal, that for the 
sreatly compressed form of the leg. Internal to it the bone is distinctly 
oO 
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