416 ME. ST. GEORGE MIVABT ON THE 



The parapophysial sm-face for the head of the rib is much as in the twenty-second 

 vertebra (fig. 52, p). The diapophysis extends somewhat less postaxially, and is less 

 antero-posteriorly extended at its distal end ; and the surface for the tubercle is some- 

 what less remote from the parapophysis. 



The neural spine is more extended dorsally, being longer in this direction than 

 antero-posteriorly. 



The prezygapophysial articular surfaces are smaller than those of the twenty-second 

 vertebra ; but the same cannot be said of the postzygapophysial ones. 



Great excavations appear on each side of the vertebra, in front of, ventrad to, and 

 behind the diapophysis- 



Thus we have a side fossa, partially filled up with little lamellse and spicula of bone, 



TWENTY-THIRD VERTEBRA (i natural size). 

 Fig. 52. 



Lateral aspect. Letters as before. 



on the preaxial aspect of the root of the diapophysis. The fossa is bounded dorsally 

 by the ridge running from the dorsal side of the diapophysis to the prezygapophysis, on 

 the ventral side by the ridge running from the ventral side of the diapophysis to the 

 parapophysis. Again, there is a great depression at the side of the neural lamina, just 

 ventrad to the diapophysis ; and the same may exist in the two preceding vertebrae. 



On the dorsum of the neural arch we see that the lateral depressions on each side of 

 the preaxial part of the neural spine have much extended. The postaxial median exca- 

 vation of the neural spine is more open, while the margins of the laminae which laterally 

 bound it form a very obtuse angle with the postaxial margin of that spine. 



Those postaxial excavations which in the two preceding vertebrse were mentioned as 

 existing on each side immediately ventrad to the postaxial margin of the diapophysis 

 may be here more conspicuous ; they may be so, in part, on account of the less develop- 

 ment postaxially of the dorsum of the diapophysis, so that these lateral postaxial exca- 

 vations may appear on the dorsum of the vertebra and as but slightly separated from 

 the preaxial lateral depressions on each side. 



The postzygapophyses either do not project at all postaxially beyond the centrum, or 

 but very shghtly so (fig. 52, pz). 



