324 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [Yol.YI. 



ceutre of the bone, above, a kuob-like tubercle acts as the neural spiue, 

 which has mesially and behind a deep pitlet for the insertion of the iu- 

 terspinous ligament. Anapophysial tubercles are found above the post- 

 zygapophyses, which latter are of considerable size, concave, and faced 

 downwards. The centrum of the axis is subcompressed from side to side 

 and supports mesially, just anterior to the second vertebral articulation 

 the lirst hypapophysis of the series. The first two segments of the ver- 

 tebral column are non-pneumatic. 



Vertebrpe throughout the chain in the young chick invariably show 

 the line of union between the centra and ueurapophyses, but it is lost 

 as soon as the birds come to be two or three months old. At this age, 

 however, still very interesting points of development are strikingly visi- 

 ble in the third vertebra, which otherwise varies but slightly from the 

 same bone in appearance as seen in the column of an old male, such as 

 Are have before us. 



The neural spine, more compressed than in the axis, is nearer the mid- 

 dle of the vertebra, still deeply pitted for the interspinous ligament be- 

 hind, and slightly so on its anterior margin. This characteristic becomes 

 sery faint among the long vertebra? in the middle of the neck, to be 

 markedly reproduced as we approach the dorsals, the posterior depres- 

 sion always being by far the best defined. We find anapophysial tuber- 

 cles still present in the third vertebra. These also exist throughout the 

 cervical series, with more or less clearness ; they form ridge-like lines 

 upon the elongated segments of the mid-neck. The zygapophysial pro- 

 cesses in general look upwards and inwards anteriorly, and vice versa be- 

 hind — the fourth vertebra having in common with the one we are now 

 describing an interzygaijophysial bar, lending to these two segments 

 that broad and solid appearance well known to ornithotomists, not pos- 

 sessed by any other of the cervicals. The neural canal in the third ver- 

 tebra is nearly circular, which is also its form in the adult, becoming- 

 only moderately compressed from above downwards in the last three or 

 four cervicals. Eegarding the third vertebra from below, we observe 

 that the articulating surface of the centrum for the axis to be quite con- 

 cave and turned a little downwards. The processes that fall beneath the 

 prezygapophyses form what would be a canal with its lateral margins ; 

 this groove, however, in the "bird of the year" is converted into the ver- 

 tebral canal by an independent ossicle being placed over it on either 

 side, and, being below the rest, it causes a broad shallow concavity to 

 appear mesially and anteriorly. 



These small bones have at the very outstart stumpy apophyses pro- 

 jecting backwards, and are the parapophyses of the vertebra — the pro- 

 jections they meet from above being the pleurapophyses, the groove they 

 form mesially being the broad termination of the carotid canal. 



The fourth vertebra has the same general appearance of the third, that 

 we have just been describing ; it is a little longer, however, and in both 

 large pneumatic foramina are found laterally and beneath the diapoph- 



