1888.] PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 257 



The Eustachian tubes seem to have a common aperture, aud the ex- 

 ternal opening is nearly naked. 



Behind this orifice the basi-temporal region is represented by a broad 

 area, which, in common with the margins of the subcircular foramen 

 magnum aud the small hemispherical and sessile condyle, is in the hori- 

 zontal plane. 



An upper view of the mandible shows it to be perfectly V-shaped, 

 with rather a short, longitudinally grooved symphysis. Its anterior 

 apex is at the intersection of the lower ramal borders, which are straight 

 lines, while the superior borders, forming as they do the sides of the 

 sympbysial -groove above, become gently convex and approach each 

 other to meet in this apex below. The anterior end of the mandible in 

 an Albatross is shaped in much the same manner. 



The side of each ramus is broad from above downward just beyond 

 the feebly pronounced coronoid process. Beyond this the upper and 

 lower borders are nearly parallel, and the space they include quite 

 narrow. 



A mandibular end is triangular in outline with the apex below, and 

 the plane of its area making but a slight angle with the plane in which 

 the inferior ramal borders are found. 



The original elements of this lower jaw are completely united together, 

 leaving scarcely any trace of their original margins or a ramal vacuity 

 where it usually occurs in many birds. 



For the rest the bone is pneumatic, the foramina being at their usual 

 sites, upon the inturned processes of the articular ends. 



Petrels have a broad first basi-branchial in their Jiyoicl arches, w T hich 

 co-ossifies with a spine-like second basi-branchial. 



The cerato-hyals and glosso-hyal never form in bone, while the sec- 

 ond arches are exceedingly delicate osseous threads in Oceanodroma, 

 curving up behind the occiput in the usual fashion of the class; these 

 "greater eornua" being composed of the common elements and articu- 

 lated in the common way. 



Of the vertebral column, etc. — Usually iu this Petrel the first free pair 

 of tiny ribs occur upon the fourteenth vertebra of the column, but in 

 one of these specimens they are liberated also on the thirteenth. In 

 either event, however, there is a long, delicate free pair suspended from 

 the fifteenth vertebra. 



Neural spines occur upon the second to the sixth, inclusive, but are 

 from thence onward suppressed until this feature makes its appearance 

 again in the fourteenth vertebra. The hyapophysial canal is found iu 

 the sixth, seventh, and eighth, but thereafter a process is found all the 

 way through the series to the sacrum. These hyapophysial processes 

 are quite prominent in mid-dorsal region. Here, too, the neural spines 

 are very intimately connected together, but nevertheless the vertebra' 

 are all movable upon one another. 



From the sixteenth to the twenty-first vertebra, inclusive, we find a 

 Proc. N. M. 88 17 J#1 fl( ,,,. 



