AXIAL SKELETOX OF THE OSTEICH. 407 



Tn front of these hypapophyses the ventral surface of the centrum presents a wide 

 and rather shallow transverse concavity, bounded by two diverging ridges, which 

 proceed respectively from the root of each hypapophysis to the adjacent parapophysis. 

 Behind the hypapophyses there is no trace whatever of a median hypapophysial ridge, 

 but the centrum widens rapidly backwards into a nearly flat slightly concave surface 

 with a rounded postaxial margin. 



In the place of styliform ribs there are conical prominences so short and stumpy that 

 their nature would hardly be suspected but for the conditions presented by more pre- 

 axial vertebrae; their direction, however, is the same as heretofore (figs. 35 & 36, ^s). 



The pleurapophysial bands of bone are still less extended antero-posteriorly, though 

 they enclose a canal for the vertebral artery, which has now become so large as nearly 

 to equal the neural canal in diameter (fig. 37, v). 



The metapophysis (figs. 35, 36 & 38 m) is very largely developed ; but the prezygapo- 

 physis extends less decidedly preaxiad of the parapophysis than in the sixteenth 

 vertebra. 



1h.e postzygapophysis also projects postaxially a little beyond the centrum. 



The prezygapophysial articular surfaces look slightly more inwards, and the postzyga- 

 pophysial ones decidedly more outwards, than in the sixteenth or preceding vertebi-se. 



The neural spine has become so wide as (it may be for the first time) to exceed in 

 transverse extent the dorsal surface of the neural laminae on either side of it. 



The anterior excavation of the neural spine (fig. 36, ns) has become a considerable and 

 open fossa, while its posterior excavation (fig. 37,/) has assumed very large proportions, 

 with a vertical preaxial wall, and laterally more or less sheltered and overshadowed by 

 the projecting margins of the ridges, which diverge postaxially from the neural spine to 

 the postzygapophyses. The neural spine itself is very little extended antero-posteriorly, 

 less than in any of the nine preceding vertebrse ; but it has a greater extension dorsad. 



One or two conspicuous foramina lead from the side of the centrum or neural arch 

 into the substance of the bone. 



CERVICO-DORSAL VERTEBRA. 



The Eighteenth Veetebea. 



The eighteenth vertebra differs as much from the seventeenth as does the latter from 

 the sixteenth. 



It is a larger bone than the seventeenth vertebra, inasmuch as, though the centrum 

 is shorter, the extreme antero-posterior extent of the whole vertebra is not less, while 

 its transverse and its dorso-ventral dimensions are augmented. 



The excess of the transverse diameter of its preaxial part over that of its postaxial 

 part is greater than even in the seventeenth vertebra (fig. 41). 



This vertebra bears the first rib. 



