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 fossae being correspondingly narrow and deep. The 

 bones entering into its composition, and supporting it become extensively pneu- 

 matic. Anteriorly the ilia unite, or nearly so, over the neural crest, whilst pos- 

 teriorly the foramina between the transverse processes of the sacral vertebras 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 Pelecano'ides the ischia are nearly straight along their posterior margin. 



In Ci/rnochorea and Procellaria, as well as in the Oceanitidas, 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 Oceanitidaa 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 Extremity. — The femur is short, and more or less curved, most so in 

 Puffinus. 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 Oceanitidse is a stronger and 

 better developed bone, particularly at the two extremities, than it is in the Procellariidaa 

 of corresponding size. The tibia is long, especially in the Oceanitidaa, 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 PI. 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 Diomedeinae 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 greatly compressed form of the leg. Internal to it the bone is distinctly 

 grooved. 



Of the articular trochlear 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 



