424 ON THE PETRELS COLLECTED DURING 



unite, or nearly so, over the neural crest, whilst posteriorly the foramina 

 between the transverse processes of the sacral vertebras become reduced 

 to small holes, owing to the increased amount of ossification. A ten- 

 dency 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 Oceanitidee, 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 Oceani- 

 tidae 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 sur- 

 face. The femur of the Oceanitidse is a stronger and better- developed 

 bone, particularly at the two extremities, than it is in the Procellariidae 

 of corresponding size. The tibia is long, especially in the Oceanitidse, 

 where it is at least twice as long as the humerus. It has a very large 

 and strocg cnemial process, best developed in the genus Puffinus, for the 

 attachment of muscles, rising high above the articular faces of the 

 femur (vide PL XXI. fig. 6). Its distal end is more or less antero- 

 posteriorly compressed, and has an osseous bridge for the extensor corn- 

 munis 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 trochleae at its distal end, the inner is the shortest and 

 most oblique. Those for the third and fourth digits are more nearly 

 Zool. Chall. equal, that for the third, however, being slightly the longer. There is a 

 Exp. vol.jv. 8ma ii foramen between it and the fourth. 



In Puffinus there is a distinct osseous bridge, developed on the anterior 



