1"-:] PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. "219 



The basi-temporal region is broad and smooth, and a spine-like pro- 

 cess at its apex fails to shut out from view the double orifice leading to 

 the Eustachian tubes. 



We find the major portion of the crotaphyte fossa upon the lateral 

 aspect of the skull. Still it may be seen also from a posterior view. 

 where the two depressions approach each other, but are separated by a 

 large dome like, supra-occipital prominence. 



This latter is usually pierced by an irregular foramen on either side, 

 which is quite characteristic but not always present in the Ducks and 

 Geese. In a specimen of Branta canadensis hutchinsii before me a large 

 one occurs only on the left side of the promiuence. 



Mergus has a large foramen magnum which faces almost directly back- 

 ward. The occipital condyle at its lower margin is of a reniform out- 

 line with the notch above. 



In the mandible the symphysis is short, and this bone, when seen from 

 a superior aspect, is of an acute V-shape form. 



The anterior two thirds of either ramus is narrow, tapering somewhat 

 to the front, with both upper and lower borders rounded. On the outer 

 surface a deep, median, and longitudinal groove of hair-like proportion 

 is drawn along its entire length. 



The hinder third is much wider, nearly double the width, and, instead 

 of being thick like the fore part of the bone, is a vertical lamelliform 

 plate. Its border is sharp above, while below it is rounded, being in 

 the same line with the inferior border of the anterior two-thirds. 



The ramal fenestra is nearly or quite closed in by the surrounding 

 elements ; a long, oblique slit marks its site. A curved projection is de- 

 veloped on the outer aspect of this part of the bone: that above appar- 

 ently takes the place of part of the corouoid process. 



Each mandibular facet presents two oblique grooves upon an area 

 contracted to the minimum extent that would accommodate the man- 

 dibular foot of the quadrate that articulates with it. 



Behind, either angle is produced backwards as a recurved and ver- 

 tical lamina of boiu-, to the inner side of which we find the circular 

 entrance to a deep conical pocket. 



Mergus serrator has an enormous bilobed tracheal tympanum at the 

 pulmonic bifurcation of its windpipe. These interesting structures 

 vary much in form and size in the different species of birds that pos- 

 sess them, and would well repay a geueral comparison. 



Of the vertebral column and ribs. — This Merganser has sixty-one ver- 

 tebrae in its spinal column ; the first pair of free ribs occurring on the 

 sixteenth; then follow five others that have ribs connecting with the 

 sternum by costal ribs; seventeen anchylos to form a sacrum for the 

 pelvic bones: and. finally, we find seven free caudal vertebra' besides a 

 pygostyle. All these segments are freely movable upon one another, 

 except those in the sacrum. In Mergus the odontoid process of the 



