472 



Prof. W. K. Parker. 



[Mar. 22, 



On the Presence of Procoelous Vertebrae in Birds, and of the Imperfect 

 and Irregular Joints between the Centra. 



The modern procoelous Reptilian form of vertebral articulation is 

 not altogether wanting in Birds. The atlas, although devoid of its 

 proper centrum, forms a more or less perfect joint of this kind in all 

 birds ; it is crescentic in many of the Precoces, and circular in most 

 of the Altrices ; and in the latter it is not notched above for the 

 " odontoid or suspensory ligament," but perforated. 



But in many of the higher or Altricial birds the last two movable 

 joints in the caudal series become procoelous, and also acquire a 

 joint-cavity. The rest of that series have a sub-concave joint, with 

 an intervertebral fibro-cartilage filling in the slight interspace ; these 

 joints, however, retain the suspensory ligament like all the rest, and 

 towards the end of this series the centrum is perforated by this 

 remnant of the notochord, as in lower types. 



The joint formed by the hind part of the atlas and fore-part of 

 the axis is irregular ; it cannot be classified with any of the other 

 modes of articulation, but this arises from the fact that it is formed 

 between the cortical or inferior part only of those two vertebras. The 

 two first vertebras are greatly modified in all the " Amniota," an 

 anticipation of which is found in the Urodelous Amphibia.* 



There are two main varieties, in Carinate birds, of the articulation of 

 the atlas with the occipital condyle, and of the atlas with the axis. 



These correspond on the whole with the Natural Division of birds 

 into "Altrices" and "Precoces"; the Piping Crow of Australia (Gym- 

 norhina tibicen) may be taken as an example of the first, and the 

 Australian Bastard (Eupodotis australis) of the second kind. 



In Gymnorhina the atlantal (procoelous) cup is a perfect hemi- 

 sphere, but near its upper rim the suspensory ligament passes through 

 a small hole to reach the basi-occipital. This cup fits well under 

 the hemispherical occipital condyle ; it is in position intermediate 

 between that condyle and the true atlantal centrum. The hind face 

 of this imperfect vertebral body is scooped so as to form a crescentic 

 groove, with the concavity upwards ; the convex fore- end of the axis 

 fits into this groove, and the atlas grows under the joint as a bilobate 

 and carinate process ; the joint is a crescentic condyle with its con- 

 cavity looking upwards. 



* In many of these there is an imperfect vertebra between that which is articu- 

 lated with the two occipital condyles ; it is evidently an atlas with an imperfect 

 neural arch, and the median and lateral elements of which become fused to form the 

 odontoid process. The perfect vertebra next following is evidently the axis, but has 

 the atlantal function of carrying the skull. See Wiedersheim, ' On Salamandrina 

 perspicillata,' Grenoa, 1875, Plates 2 — 4, and my papers, " On the Skulls of the 

 Urodeles," ' Linn. ooc. Trans.,' Ser. 2, Zool., vol. 2, Plates 14 — 21, and 'Zool. Soc. 

 Trans.,' vol. 9, Plate 40. 



