SHUFELDT : OSTEOLOGY OF THE STEGANOPODES 139 



as the sclerotal plates, the tongue, or rings of the trachea, or much less certain 

 sesamoids we will hereafter he called upon to describe, are ever pneumatic. 



Of the Remainder of the Skeleton of the Irunk in the Sulidse. (See Plate XXIII., 

 Fig. 12.) 



In the common Gannet there are twenty-one free vertebne in the spinal column 

 before we meet the one that first anchyloses to form, with the assistance of the four- 

 teen succeeding ones, a sacrum for the pelvic bones. Then follow seven more free 

 ones, devoted to the movable part of the tail. Finally, we have a long pygostyle 

 that probably contains at least six more. Owing to the lengthening behind of its 

 pelvic bones, the sacrum contains sixteen vertebrse in SuJa cyanops, and that species 

 has but six free caudal ones plus the pygostyle. 



The sixteenth, seventeenth and eighteenth vertebrse support each a pair of free 

 ribs ; the next three belong to the dorsal series, and all have true vertebral ribs 

 articulating with costal ribs from the sternum. This is also the case with the first 

 three pairs that spring from the pelvic sacrum. The ribs on the sixteenth vertebra 

 are exceedingly rudimentary, and the last pair of " floating " costal ribs have no cor- 

 responding pair of vertebral ones above them. 



In mid-series these ribs support epipleural appendages, attached in the usual way 

 to their posterior borders. (They may be anchylosed or free.) As I have already 

 stated above, they are completely non-pneumatic. 



The neural canal is notable for being nearly cylindrical throughout the first 

 twenty-one vertebrae. It is only at the region of the enlargement for the brachial 

 plexus that it is rather compressed in the vertical direction. 



The atlas has a minute perforation in its cup, and its neural arch is strikingly 

 broad and deep. The axis vertebra possesses a stumpy neural spine, and its hypa- 

 pophysis, directed somewhat backward, is very prominent. 



The odontoid peg is comparatively small and nearly sessile with the centrum, 

 the latter presenting a concave face below it. 



From the third to the fourteenth vertebra, inclusive, the neural spine is a very 

 inconspicuous character, while from this on it gradually makes its appearance, increas- 

 ing in size until we have the usual quadrate, longitudinal plate of the dorsal series. 



The third and fourth vertebrse have each a prominent hypapophysis like the 

 one on the axis, but in the fifth this feature nearly entirely disappears. 



The sixth vertebra is faintly marked by the carotid canal ; this gradually be- 

 comes more and more tubular in the seventh and eighth, while in the ninth to the 

 thirteenth inclusive it is a closed cylindrical canal of a caliber somewhat less than the 

 neural canal canal above it. It disappears entirely from the fourteenth vertebra. 



