CACOPS ASPIDEPHOHUS 263 



plate 14, figure 11. This vertebra seems to be a single element, though 

 doubtless it is composed of coalesced hypocentrum and neurocentra; but 

 I can distinguish no sutural lines. The anterior surface shows two facets 

 for articulation with the occipital condyles. On the posterior side the 

 body has a deep concavity, pierced above its middle by a small noto- 

 chordal foramen. The neuropophyses are simple processes, of nearly 

 uniform width, and flattened; they lie closely in apposition with the 

 sides of the spine of the second vertebra. The same condition is found 

 in Eryops and Trematops, and is doubtless the usual structure of the 

 atlas in the rhachitomous amphibians. Back of the neurocentra I find no 

 articular surface for the attachment of pleurocentra, though the anterior 

 border of the next vertebra seems to indicate the presence of small pleuro- 

 centra. 



The real composition of the atlas of the rhachitomous amphibians — 

 for I doubt not that the vertebra is homologous with the atlas of the 

 higher vertebrates — is a matter of some importance. If there be pleuro- 

 centra between the atlas and second vertebra, then we have the same 

 structure as exists in the reptiles and higher vertebrates, the atlas com- 

 posed of hypocentrum and neurocentra, the pleurocentra separated to 

 unite with the axis. Against this interpretation, however, is the fact, as 

 seen in the drawings, that the notochordal opening pierces the centrum 

 precisely as it does in the later vertebra — ^that is, between the pleuro- 

 centra and hypocentrum. If all three elements are coossified in the atlas, 

 then it would lend support to the views held by some morphologists, of 

 whom Broili has given the latest exposition, that the vertebra of the 

 amniota are composed of the combined hypocentrum, pleurocentra, and 

 neurocentra. 



The second vertebra of tli^ series, which we may call the axis, because 

 of its slight modifications in structure, has a smaller h3'pocentrum, as I 

 have described, without parapophysial facets. The diapophyses are short 

 and narrow at their extremity. The spine is much broader below than 

 are the succeeding ones and pointed above, where it comes in contact at 

 its extreme tip with the extreme front end of the carapace. On either 

 side of the thinned anterior expansion of the spine below there is a slight 

 depression, in which is lodged the slender flattened neuropophysis of the 

 atlas. 



SACRUM 

 (Plate 9) 



As is well known, the reptiles and higher vertebrates, wherever thev 

 possess a sacrum, have invariably, or almost invariably, two or more 



