296 OSTEOLOGY OF TUBINARES AM) s TEGANOPODES. 



tween the trochlea of the mandibular foot of the quadrate, creating an 

 irregular hollow space there of no inconsiderable size between the 

 bones. When the quadrate thus covers it there are two entrances that 

 are left open, one in front and one behind, close to the pneumatic for- 

 amen. 



The mandibular angles are truncate and very nearly perpendicularly 

 SO, their surfaces being concave and very broadly luniform in outline. 



Commencing just in front of an articular cup, we find the superior 

 border of the ramus to be rather wide and rounded as far as the meet- 

 ing with the dentary . This portion presents near its middle a double 

 corauoid process, one being in front of the other. The dentary portion 

 of this border has an outer cultrate edge and an inner and somewhat 

 lower rounded one. 



The outer edge goes to the anterior apex of the symphysis, the inner 

 one to the hinder termination of the same, while between the two a 

 nearly horizontal surface is contained, which gradually becomes nar- 

 rower as we proceed in the forward direction. 



The lower borders of the mandible are rounded for their entire ex- 

 tent, being produced beneath the articular cups and continuous with 

 the inner boundary of either truncate angular extremity. 



We fiud that the usual bones which surround the true rainal vacuity 

 on the side of the mandible in many birds here interlock with each other 

 so as to completely fill the fenestra in, but in rather an unusual way 

 and apparently for a definite purpose; for each ramus presents, both 

 on its inner and outer side, an oblique slit, these slits being opposite 

 each other and with their anterior ends in the superior border. It is 

 evident that this otherwise thick jaw is much weakened at these points 

 in each ramus, and this occurs just posterior to the hinder termination 

 of the homy sheath of the lower beak. In other words, the hinder 

 moities of the mandible are attached to the anterior or dentary portion 

 by thin plates of bone, consisting principally of the splenial elements, 

 and are capable of beiug bent outward, which in the recent specimen 

 can, owing to the way the quadrates are attached, be effected to a con- 

 siderable degree. Now in life these oblique slits have their anterior 

 ends come opposite the thin anterior insertions of the maxillaries, and 

 these latter are just beneath the very mobile cranio-facial hinge, so 

 that the whole apparatus is admirably arranged to permit an increase 

 in size of the fore part of the buccal cavity when this Ganuet swallows 

 the fish that constitutes its food, and which its beak is so well fitted 

 otherwise to capture. Moreover, this possible increase in caliber tabes 

 place in that portion of the digestive tract where it is most needed, or 

 where the bony walls of the mouth would prevent the admission of a 

 very large morsel unless some such mechanism existed— at the very 

 entrance of the buccal cavity and just posterior to the more horny 

 thecae of the beak. In Gannets, however, this mobility is to an extent 

 restricted by the integumental sheath of the beak. 



