598 5. W. WILLISTON 



considerable interval both the arches and the hypocentra of the 

 adjacent vertebrae. This pleurocentrum forms a complete ring, 

 without traces of division, conically hollowed in its visible end and 

 perforated by the notochordal canal. Its neurocentrum is much 

 more closely and extensively combined with it than with the pre- 

 ceding pleurocentrum. In a few words, this vertebra is still typi- 

 cally rhachitomous, save that its fused pleurocentra form a disk 

 separating the adjacent vertebrae, which is perforated like any 

 holospondylous centrum by the notochord. The most imaginative 

 eye will not see in this vertebra fused pleurocentra and hypocentrum 

 with some other element taking the place of the hypocentrum, 

 since the preceding apparently fused pleurocentra are not very 

 different from the ordinary form. Doubtless the pleurocentra 

 preceding these were progressively smaller, and those following 

 progressively larger. To follow Jaekel's arguments to their extreme 

 would necessitate the fusion of hypocentra and pleurocentra 

 throughout, and the sudden introduction of an entirely different 

 element in the chevrons to mimic the hypocentra, of all of which 

 there is not the ghost of evidence! 



The next stage in the evolution of the ordinary holospondylous 

 vertebra may be seen in the reptile Desmospondylus, as recently 

 described and figured by me,^ an outline copy of two of the vertebrae 

 of which I reproduce (Fig. 3, C) . In his specimen it will be seen that 

 the fused pleurocentra (PP) have increased in size, while the hypo- 

 centra (HH) have decreased, though still much larger relatively 

 than in any other known reptile. The arch (N) rests in the same 

 way upon the two adjacent pleurocentra, though functionally 

 upon the posterior one, its own, and its lower extremity in front 

 nearly touches the upper extremity of the hypocentrum. 



From these three specimens it is not at all difficult, it seems to 

 me, to understand clearly the way in which the different types of 

 vertebrae have arisen. By the fusion of the neurocentrum with 

 its respective hypocentrum, the embolomerous vertebra has arisen; 

 by its sutural union with its respective pleurocentra the reptilian 

 vertebra is produced; by the union of all three, I believe, the 

 holospondylous amphibian vertebra has been evolved. It would 



I Bull. Geol. Soc. Amer., XXI (19 10), 280. 



