146 



at the fore part of the base of the spine, on an eminence, answering to the zygosphene in 

 Serpents, and two depressions at the back part of its base, or zygantra, for receiving the 

 accessory zygapophyses of the following vertebree. The vertebrae of the Iguana thus present 

 what has been regarded as the character of those of Serpents, and the difference is one of 

 degree rather than of peculiar structure ; the zygosphenal surfaces are relatively smaller, and 

 the zygantra are less deep. The much smaller proportions of the tubercular diapophyses for 

 the ribs is another character by which the vertebra of the Iguana may be distinguished from 

 that of the Serpent. The depressions on the under part and sides of the body of the verte- 

 brae in the Iguana which give rise to the three low ridges diverging from the posterior ball, 

 one to the cup and one to each diapophysis, is another feature of resemblance to the vertebras 

 of Serpents which is present in the vertebree of the Iguana and not in those of the Varanus. 



Presented by Prof. Owen, F.E.S. 



669. The six anterior vertebra of an Iguana (Iguana tiibercttlata). 



The autogenous hypapophysis of the atlas is broad, like a wedge with the base divided into 

 four parts, the apex forming a sharp longitudinal ridge produced backwards : the anterior, 

 upper or basal facet forms the lower part of the cup for the occipital condyle ; the posterior 

 facet joins the proper centrum of the atlas or odontoid process : the two lateral facets support 

 the neurapophyses of the atlas : the lower half of the neurapophyses forms anteriorly the sides 

 of the cup for the condyle, and posteriorly each sends out a short diapophysis which supports 

 a cartilaginous tubercle or rudimental parapophysis. Above this, each neurapophysis arches 

 over the neural canal in the form of a broad and thin plate which meets its fellow without 

 coalescing or developing a neural spine, but sends out from its back part a zygapophysis sur- 

 rounded by a tubercle. The odontoid process resembles the hypapophysis of the atlas in 

 shape, except that its apex forms a slender spine instead of a ridge, and its base rises higher, 

 to form the back part of the cup for the occipital condyle. The proper body of the axis has 

 coalesced with the neural arch ; it developes on each side of its fore part a broad, short diapo- 

 physis, overlapped by that of the atlas ; its hypapophysis is a triangular epiphysis wedged into 

 the under part of the interspace between the axis and third vertebra. The under part of the 

 axis is sharply keeled : the neural arch sends off two prezygapophyses looking upwards, and 

 two postzygapophyses looking downwards, and forms above a long and strong sharp spinous 

 ridge, continued forwards over the atlas and backwards over the third cervical vertebra. The 

 diapophyses of this vertebra support a minute cartilaginous tubercle, and a ridge is continued 

 backwards from each, which bounds the concavity on the under and lateral part of the cen- 

 trum divided by the median carina. The hypapophysis has the same shape, independence, 

 and relative position as that of the axis. Besides the ordinary zygapophyses, there are two 

 smaller and superior ones developed from the fore-part of the base of the neural spine, which 

 is comparatively slender and trihedral : the postzygapophyses have a nearly vertical facet upon 

 their inner sides, lodged, as in the axis, in an excavation beneath the back part of the base 

 of the neural spine. 



The fourth cervical resembles the third, but its hypapophysis is smaller. In the fifth cer- 

 vical it is further diminished, though still distinct, and wedged into the lower angle between 



