Dr. Francis Baron Nopcsa— British Dinomurs. 245 



dorsals and lumbars, and in consequence have an abbreviated 

 appearance. The processus transversi are given off from the 

 centrum, and are pointed nearly horizontally outwai'ds. A similar 

 abbreviated and expanded structure of the centrum may be observed 

 on some, not yet described, Bacenturus-lxke vertebrge in the Havre 

 Museum. I therefore think the original shape of the proximal 

 caudals in Polacanthus has been only slightly disfigured by pressure. 

 The neural spine is not very long, but very thick, and the pree- and 

 low pree- and postzygapophyses do not project far out over the 

 articular surfaces of the centrum. 



The median caudals differ by having a less depressed, less 

 expanded, and more elongate centrum, and the diapophyses directed 

 downwards and outwards. The neural canal is much smaller, the 

 chevron bone articulated nearly entirely with the hind part of the 

 centrum, there being scarcely any impression for its articulation on 

 the anterior margins. 



3 4 



Fig. 3. — Proximal caudal of Polacanthus, posterior view. 

 ,, 4. — Middle caudal of Polacanthus, anterior yiew. 



Further back, towards the end of the tail, the vertebra become 

 «till more elongate, and the front articular surface, which was slightly 

 concave in the anterior and median caudals, becomes decidedly convex. 

 The posterior surface remains concave throughout the whole vertebral 

 column. In Figs. 4 and 5 sketches of middle and posterior caudals 

 are given. 



Until now only one chevron bone of Polacanthus has been detected, 

 but this one is quite characteristic. The top view shows a nearly 

 quadratic section, and at its distal end its thickness is about twice 

 that of its antero-posterior dimensions. Both rami are exceedingly 

 strong, and in accordance with the general massive structure of the tail. 



There is a striking resemblance between the root of the tail of 

 Polacanthis and that of the gigantic Megatherium ; the rest of the 

 tail has, however, been reconstructed after the Hylceosaurus pattern. 



