of the Neural Arches in Ichthyosaiiria. 443 



arclies rest upon eacli other and interlock. Tlie prezyga- 

 pophysis is ly^o inch deep and fg inch wide, flat, vertically 

 ovate, and not appreciably raised above tlie level of the 

 oblique zyf^apophysial surface, which extends backward above 

 the neural canal, to the base of the neural spine. It is about 

 /o inch above the neuro-central suture (tig. 2, A). The 



Fig. 1. 



F\i 



Fig. 1 . — Lateral aspect of the neural arch in Ophthalmosaurns 

 Fig. 2. — Ophthalmosaunis iceuicus : neural arch. A, anterior 

 rior. Half natural size. 



poste- 



correspondiiig posterior zygapophysial facet is parallel and 

 entirely behind the anterior facet (tig. 1), but the vertical 

 distance between the back of one and the front of the other 

 is about tV inch. This posterior facet is a little longer and 

 a little wider (fig. 2, B), giving a slight bulge to the posterior 

 border of the neural .^piue at the articulation (tig. 2, A). 



The neural spine is compressed from side to side, three to 

 four tenths of an inch thick above these articular facets, and 

 becoming a little thicker towards the free end. It is limited 

 back and front by a sharp ridge. The antero-posterior 

 measurement between these borders is 1-]% inch above the 

 postzygapophysis, to l/j at the free termination. The 



