Vol. 56.] AKISTODESMUS RtJTIMEYERI. 625 



concave back and front from within outward, and compressed at the 

 anteroinferior margin into a ridge which descends upon the centrum 

 towards the middle of its articular border. All the characters of 

 the transverse processes of Pareiasaurus are intensified in this 

 animal, so far as comparison can be made. 



The neural arches of the vertebrae have a quadrate form, and the 

 neural spine is vertical. 



The most striking feature of the neural arch is the transverse 

 process, which gives attachment to the rib. These processes extend 

 transversely outward, so as to be horizontal, immediately behind the 

 pre-zygapophyses, and slightly in advance of the middle lengtb of 

 the vertebra. A notch which indents the side of the vertebra, 

 between the posterior zygapophyses and this process, defines its 

 posterior side. Its anterior border, also vertical, is defined by 

 extending beyond the pre-zygapophyses. As exposed, these articular 

 surfaces, which terminate the transverse processes, are nearly vertical 

 truncated facets, slightly concave and converging a little inferiorly. 

 There is no indication of a transverse division between the part of 

 the articulation which is upon the neural arch and that upon the 

 centrum ; but as the articulation on the centrum is appreciably 

 narrower, there may be an angle between the two portions of the 

 rib-facet. 



The pre-zygapophyses extend forward from the front of the trans- 

 verse processes like jutting ledges. Seen from above they are sub- 

 quadrate, horizontal, each a little wider than long, and approximate 

 towards each other at the base of the neural spine. The facets are 

 defined by a concave notch between them. The buttresses beneath the 

 facets are margined by lateral ridges. The neural canal is of moderate 

 size. 



The neural spine is vertical, triangular, and strong without being 

 massive. The posterior zygapophyses are well defined, compressed, 

 rounded ridges, which continue the posterior angles of the triangular 

 neural spine downward, outward, and backward. The concave 

 posterior surface of the neural spine becomes a wide notch between 

 the posterior zygapophyses. The summit of the neural spine is 

 truncated and rounded. 



Compared with Pareiasaurus bombidens^ the stronger transverse 

 process is more elevated. 



There may be some approach in form of neural spine and transverse 

 process to Protoresaurus, though the preservation of the specimens 

 of that genus is less satisfactory. 1 Another approximation is seen 

 in Nothosaurus, in which the rib has a deep vertical articular facet 

 on the vertebra. The resemblance derives some interest from the 

 structure of the Nothosaurian shoulder-girdle. 



The Ribs. 



In his restoration Prof. Wiedersheim represents twenty-one 

 pairs of pre-sacral ribs, while there are no ribs to the first vertebra. 



1 This is not supported by the figure given bv Franz Etzold, Neues Jahrb. 

 1898, vol. ii, p. 148. 



