AXIAL SKELETON OF THE OSTEICH. 



411 



smaller excavation (either several small openings or one large foramen) is placed a little 

 preaxially of the postaxial margin of the neural lamina, on the ventral side of the 

 interzygapophysial ridge. 



The neural spine has its dorsal margin more antero-posteriorly extended than in the 

 eighteenth vertebra ; and consequently the margins of the ridges running from it to the 

 postzygapophyses, instead of sloping gradually, run at first a little preaxially, thus 

 forming a slightly acute angle. 



The postaxial excavation of the neural spine is much as in the eighteenth vertebra, 

 though somewhat less deep. The preaxial surface of the neural spine is vertical, but 

 cannot always be said to be medianly excavated; rather there is sometimes a slight 

 depression on each side of it on the neiu'al lamina. 



The liirezycjapoplvyses are broader than in the eighteenth- vertebra, and look rather 

 more inwards. The postzygapophyses are smaller and look even more outwardly. For 

 the first time their transverse (here actually dorso-ventral) diameter may exceed their 

 antero-posterior development ; and they are so produced that their articular surfaces may 

 be altogether postaxial to the postaxial margin of the centrum. The preaxial margin 

 of the neural arch begins to show a little median preaxially directed process (fig. 44). 



The Twentieth Vertebea. 



The twentieth vertebra closely resembles the nineteenth, with the centrum, diapo- 

 physes, and neural spines slightly enlarged. 



The centrum has its preaxial articular surface relatively deeper dorso-ventrally ; and 

 its lateral parapophysial terminations are not so much laterally produced. 



TWENTIETH VERTEBRA (i natural size). 

 Fig. 4.5. Fig. 46. 



Fig. 45, lateral aspect ; 46, postaxial aspect. Letters as before. 



The pre- and postaxial diapophysial excavations of the neural lamina are the same as in the nineteenth 



vertebra ; only the postaxial excavation is relatively the more extensive. 



The postaxial surface has its ventral margin more convex, but relatively shorter 

 compared with its dorsal margin, than in the nineteenth vertebra ; the whole articular 

 surface begins also to be less convex transversely and less concave dorso-ventrally. 



3m2 



