122 MEMOIRS OF THE CARNEGIE 



free, second basibranchial. Either ceratobranchial is an extremely long, slender rod 

 of bone, bearing at its distal end a little bit of an epibranchial, which latter is fin- 

 ished off posteriorly by a thread-like extension in cartilage. Cartilage also tip the 

 glossohyal in sfront, and laterally, the ends of the ceratohyals may not entirely ossify. 



For the trachea, its rings seem to be performed in elementary osseous tissue only, 

 and in mid-course of this tube, they are delicate structures, being markedly narrow 

 in depth, and very frail. 



Of the Axial Skeleton. — (See Plate XXI., Figs. 5 and 6.) Representatives of the 

 genus Phaethon possess in the cervical and cervico-dorsal divisions of the vertebral 

 chain 15 bones. Three of these are cervico-dorsals, the anterior one of which 

 always supports an exceedingly rudimentary pair of free riblets. 



In P. sethereus they are seen to be in the same condition on the next following 

 vertebra, or the 14th vertebra of the neck, while in P. flavirostris they are of some 

 considerable length, though they do not develop epipleurse. Both species have a 

 well-developed pair of ribs on the 15th segment of the neck vertebra, that support 

 epipleural spines on their posterior margins, to which part they are firmly coossified. 



The atlas is comparatively small with rather slender neurapophyses. Its cup is 

 profoundly notched above, for the accommodation of the odontoidal process of the 

 axis. Below, it develops a low hypapophysial spine projecting backwards, which, 

 when the bones are articulated in situ, underlaps the centrum of the second vertebra. 

 A prominent, quadrate hypapophysis also is found to exist beneath the centra of the 

 axis and third vertebra, but this feature is very nearly aborted on the fourth cervical. 



On the axis vertebra we find a massive neural spine, with conspicuous lateral pro- 

 jections, each jutting upwards and outwards from above the postzygapophyses. All 

 these characters are moderately reduced in the third vertebra, while in the fourth 

 the neurapophysis is very much reduced in size, and the aforesaid lateral or ana- 

 pophysial processes are all but absent. 



The lateral vertebral canals are but semi-closed in the axis ; they are thoroughly 

 so throughout the rest of the vertebral chain until, we meet with the leading cervico- 

 dorsal, wherein the existence of a pair of rudimentary free pleurapophyses leaves 

 these canals open. 



On the under side of the cervicals, the passage for the carotid canal is seen to 

 be wide open, and is confined to the 5th, 6th, 7th and 8th vertebrse. Throughout 

 the series, beginning with the third cervical, we find the parial backward projecting 

 spines of the parapophyses to be pointed, and exceedingly stumpy and very short. 



On the 9th a median hyapophysis makes its appearance, which is larger on the 

 10th, more quadrilateral in form and laterally compressed on the 11th, small on the 



