PLTOSATJRUS. 13 



shaped and extend from one end of the centrum to the other. From the outer angle 

 of these facets a slight ridge runs down the side of the centrum, becoming continuous 

 below with the low prominence which bears the diapophysial and parapophysial 

 surfaces for the articulation with the ribs. These surfaces taken together are pear- 

 shaped, the rounded end being at the bottom, the pointed one at the top ; they are in 

 close apposition, being divided only by a slight groove which terminates at each end 

 in a notch. The ventral surface of the centrum between the rib-facets is convex from 

 side to side and slightly concave from before backwards. There is a pair of large 

 nutritive foramina, the openings being about halfway between the mid-ventral line 

 and the rib-facets. 



The neural arch is high, the pedicles being relatively long ; they widen out con- 

 siderably at their lower end. The anterior and posterior zygapophyses (a.z. and p.z.) 

 are large ; their articular surfaces are oval and are slightly concave and convex, 

 respectively, from side to side, and look forwards and backwards to a considerable 

 extent. In this region the anterior and posterior zygapophyses are united by a ridge 

 running upwards and backwards. The neural spine (n.sp.) is high and not very broad 

 from before backwards ; at the same time it is of considerable thickness from side to 

 side, being much less compressed laterally than in the Elasmosauridae. The anterior 

 border of the lower part of the spine is thin and sharp-edged ; it runs down to the 

 anterior border of the arch between the anterior zygapophyses. Higher up the 

 anterior border becomes thickened and rounded, and at the summit widens out and is 

 roughened. The posterior face of the spine is flattened above and concave from side 

 to side below, the concavity between the posterior zygapophyses being very deep and 

 having deep pits for the attachment of ligaments. 



The dorsal vertebrae are badly preserved in all specimens. It can be seen that the 

 centra were much longer than in the cervical region, the longest being probably from 

 about the middle of the back, where the articular faces are nearly circular in outline 

 and very slightly concave ; the sides of the centra are strongly concave in a longi- 

 tudinal direction. The surfaces for union with the neural arch are very long, 

 extending nearly the whole length of the centrum ; the neural surface between 

 these facets is very narrow. About two centimetres on either side of the middle 

 line is a nutritive foramen, and there is another pair situated high up on the side. 

 No good specimen of the neural arch in this region is available for description. It 

 can, however, be seen that the pedicle of the arch was stout and that the transverse 

 process was thick and bore a broad facet for the rib at its thickened extremity. 

 The roof of the neural arch is broad and the stout neural spine is situated towards 

 its hinder portion. The anterior border of the neural spine is continuous below with 

 a deep groove which extends to the anterior border of the arch ; its edges are raised 

 and it encloses a roughened surface for the attachment of ligament. The lower end 

 of the posterior border of the neural spine is also deeply hollowed. 



