PLATANISTA. 531 



The fourth vertical has a thin, antero-posteriorly compressed centrum like the 

 third, hut it is relatively higher, its articular surfaces heing as high as broad. The 

 anterior surface is rendered deeply concave in its lower half by the forwardly pro- 

 jecting inferior transverse processes which are first strongly marked in this vertebra. 

 The laminae are very narrow and rod-like, with a very minute process at their union 

 and a roughened surface external to it, there being no spinous process. The neural 

 canal is broadly triangular and not so acutely arched as in the preceding vertebra. 

 The zygapophyses become more removed from each other, the anterior being at a 

 lower level than the posterior, and on the right side the two are separated from each 

 other by a deep notch, while on the left they are closely applied to each other due 

 to sinistral asymmetry, the left lamina of the neural arch being also dragged 

 to the left side. The relative proportions also of the zygapophyses are reversed in 

 this vertebra from that which prevails in the vertebra before it, the anterior being 

 about half the size of the posterior zygapophyses. The in vertebral notch is not so 

 broad as in the third, but it is better marked, the equivalent of the process that 

 defined its inferior outer limit in the latter vertebra is very strongly developed in 

 this one ; and, owing to the shortening of the superior transverse process, it occurs 

 at its extremity, which is thus bifurcate. The superior transverse process is short 

 and compressed from before backwards, concave on its superoposterior surface, 

 bifurcate and directed outwards and slightly backwards, lying below the level of its 

 fellow behind it and a little above its fellow before it. The base is prolonged down 

 one-half of the side of the centrum, where it is separated by a semi-circular notch 

 from the inferior transverse process. The latter has a broad base reaching from the 

 superior process to the inferior border of the centrum, tapering to a point and 

 directed forwards, its inferior surface more or less concave, and its superior surface 

 flat, its anterior surface being grooved. It is considerably smaller on the left than 

 on the right side. On the laminae, halfway between the posterior zygapophyses 

 and their line of union, a small, backwardly-projecting process simulating a hyper- 

 apophysis occurs. 



The fifth resembles the fourth vertebra in its general form, but the neural arch 

 is much stronger, and on its left side there is a stout but small backwardly projecting 

 process like an hyperapophysis ; on the right side this structure is imperfectly deve- 

 loped. The neural canal is twice as broad as high, but its two halves are unequal, 

 as the lamina of the right side is depressed . The zygapophyses of the two sides 

 are unsymmetrical, those of the right having their articular surfaces on nearly the 

 same plane, separated externally by a very small notch ; while those of the 

 left are widely apart from each other and smaller than those of the right side. 

 The transverse process, bifurcate at its extremity, is larger than in the preceding 

 vertebra, and separated by a wider notch from the anterior zygapophyses than in 

 the fourth vertebra. This process is posterior and superior to the transverse process 

 of the fourth vertebra, and lies on the same plane with the superior transverse process 

 of the sixth. The inferior transverse processes are more strongly developed than in 

 the fourth, but are considerably smaller than the superior processes. 



