1899.] OSTEOLOGY Of THE PIGOPOBES. 1033 



iv. The Vertebral Column. 



The vertebrae seem to resemble those of the Steganopodes more 

 nearly than of any other group, and, amongst the Steganopodes, 

 they most nearly approach those of Phalacrocorax. They are quite 

 different from those ot the Impennes or Tubinares. They can, 

 however, be at once distinguished from those of Phalacrocorax, 

 in that the thoracic vertebrae are heteroccelous ; but they differ also 

 in other respects. 



The odontoid ligament of the atlas is not ossified. The neural 

 arches of the anterior cervicals are not, like those of the Impennes 

 and Tubinares, deeply notched posteriorly. In the Colymbi they are 

 sharply truncated and very broad, in the Podicipides they are, as 

 in Phalacrocorax, marke.l by a slight notch ; this notch, however, is 

 cut out of the coalesced bases of u pair of hyperapophyses and lies 

 behind the postzygapophyses ; ordinarily such a notch is formed 

 by cutting away the neural arch itself so as to leave the post- 

 zygapophyses as a pair of articular surfaces, each at the termination 

 of a A-shaped fork. The hyperapophyses of these vertebrae in the 

 Divers take the form of stout pillars, grooved at the top. In the 

 Grebes the pillars become mere tubercles placed close together and 

 deeply grooved superiorly. In this they resemble the vertebrae 

 in the same region of Phalacrocorax. The neural arches of the 

 posterior cervicals do not present any very noticeable features. 



In the Colymbi the 5th to the 10th vertebrae bear catapophyses, 

 which, rapidly converging, give place to hypapophyses. These 

 run backwards to the extreme end of the centrum in the form of a 

 strong median keel. The vertebrae l-4and 1 1-13 bear catapophyses. 

 In the Podicipides the cervical catapophyses from the 3rd to 13th 

 vertebrae form deep tubular grooves for the carotids, recalling 

 those of Plotus and Phalacrocorax. The cervicals 1-3 and 16-23 

 bear well-developed hypapophyses. 



The thoracic vertebrae — and the last cervical — in the Divers are 

 all free, save the last, which is anchylosed with the synsacrum. 

 1 to 5 bear median hypapophyses, with broadly expanded free ends, 

 as in Alcidse and some Impennes, e. g. Pygoscelis. 



In the Grrebes the last cervical and the thoracics 1-4 are anchj^- 

 losed to form one mass ; the .5th thoracic is free, but the 6th and 

 7th are fused with the synsacrum. 



The synsacrum of the Pygopodes is remarkable for the extra- 

 ordinary lateral compression which it has undergone, accompanied 

 by an almost complete suppression of the di- and parapophysial 

 elements. Pleurosteal elements appear to be wanting. 



i'rom the evidence obtainable from the synsacral region of a 

 nestling Grebe we may perhaps be justified in holdiug that 

 the synsacrum of the adult includes some 15 to 17 vertebrae. Of 

 these the 1st is thoracic, the next 4 are lumbar, then follow 3 

 lumbo-sacral, 2 sacral, and 5 or 6 caudal. The 3rd and 4th lumbar 

 bear small nipple-like parapophysial processes at the base of the 

 ueuron,behindthesefollow,as just stated, 3 lumbo-sacral and2 sacral. 



