NECTURUS MACULATUS. 



399 



of which survives but two or three vertebrae, while the ventral one continues for two or 

 three more. Pari j^ansn with the loss of the transverse processes, the zygapophyses 

 become reduced and finally disappear. With the loss of these lateral elements the 

 vertebrae become narrowed from side to side, and, as this appearance is further increased 

 by the great extension of neural and haemal arches and spines, the entire column becomes 

 strongly compressed laterally, thus corresponding to the change in external form which is 

 so apparent in the living animal. In this more characteristically caudal region (circa 

 between 28 to 40) the hour glass shape of 



the centra becomes more apparent and 

 in the reduction which follows towards 

 the tip, this element becomes more and 

 more conspicuous as the main portion of 

 the vertebrae. Thus, after about the 

 4()tli vertebra, rapid reductions take 

 place in the arches, both neural and 

 haemal, and the terminal one or two 

 vertebrae consist of centra alone, the 

 posterior end of the final one being 

 rounded and without the cliaracteristic 

 cup. In this loss of arches it seems that 

 the neural arch disappears first, as in all 

 the cases examined several of the last 

 vertebrae consisted of body and haemal 

 arch alone, but the specimens examined 

 were too few in number to establish this 

 as a law. From about the point at which 

 the vertebrae first become laterally com- 

 pressed, or more exactly, after the loss of 

 the zygapophyses, there is shown an 

 almost complete dorso-ventral bilateral 

 symmetry, so that in an isolated vertebra 

 it becomes extremely hard to distinguish 

 between the neural and haemal arches. 

 There is, however, a slight difference 

 which, though (hfficult of formulation, 

 may be perceived after a little study 

 and seems to consist mainly in a some- 



id) 



Fig. 5. Caudal vertebrae ; (a) and (h) two example.s of ter- 

 minal vertebrae ; (c) instances of double haemal spines ; (d) 

 and (e) two successive vertebrae in tlie same series showing 

 doubling of neural spines. Tlie arrows in (d) pa,ss through 

 llie neural and haemal canals. 



