104 MARINE REPTILES OF THE OXFORD CLAY. 



diapophysis (tubercle) being tbe shorter and stouter and terminating in a large concave 

 facet. The parapophysial process projects considerably beyond the diapophysial, and 

 terminates in a smaller convex facet (the head) ; the two processes may be more or 

 less united by a thin plate or web of bone. Further back in the series (text-fig. 39, G, H) 

 the diapophysial facet becomes a mere step on the posterior border of the rib and 

 articulates with a facet on the outer end of the transverse process. The body of the 

 middle dorsal ribs is rather strongly curved ; its anterior face is convex, the posterior 

 flat or slightly concave ; it widens out considerably at its distal end and is strongly 

 compressed from before backwards. The hindmost rib is a sickle-shaped blade of 

 bone, curving strongly forwards ; at its inner end is the facet for the parapophysis, 

 while a small surface on its posterior border united with the diapophysis. In the 

 posterior dorsals of these Crocodiles the parapophysial and diapophysial facets do not 

 approach one another to the same degree as in recent forms, and even in the last 

 dorsal are separated by a considerable interval. 



There were probably at least two lumbar vertebrae bearing no ribs. 



The form and arrangement of the sacral ribs (PL VI. figs. 6, 6 a) in Steneosaurus are 

 very similar to those of Mycterosuchus, and reference may be made to text-figures 51—53. 

 The anterior sacral rib unites at its inner end with the anterior portion of the first sacral 

 centrum, and as described above may help to form the surface for articulation with the 

 last lumbar. The rib, as a whole, is somewhat compressed from before backwards 

 except at its outer end ; its upper edge is strengthened by a strong ridge continuous at 

 its inner end with the overlapping process from the neural arch. The anterior face of 

 the rib is nearly vertical or sloping a little backwards; externally the bone expands 

 greatly and bears a large iliac surface which is deeply concave, rugose, and roughly 

 triangular in outline, the lower and antero-superior angles being rounded off, 

 while the postero-superior angle is truncated by another surface which looks directly 

 backwards and unites with a corresponding surface on the front of the second sacral 

 rib. The inner end of this latter is firmly united with the side of the second sacral 

 centrum, and to some degree with the base of the neural arch. At its outer end it 

 is much thickened, the thickened portion being compressed so that its anterior face 

 looks upwards and forwards, the posterior downwards and backwards, and at the 

 same time its postero-superior border is produced into a thin flange of bone, which 

 in most specimens is broken away ; at the outer end there is a deeply concave surface 

 fitting against the hinder part of the inner face of the ilium, and a forwardly directed 

 triangular surface for union with the anterior sacral rib. 



In the type specimen of Steneosaurus obtusidens it appears that the rib of the 

 anterior caudal is thickened and expanded like the sacral ribs (text-fig. 50), and 

 although it may not have actually united with the ilium, it joined the outer end 

 of the posterior sacral rib, which bears a facet for this union. In this case 

 therefore there were practically three sacrals, a condition, so far as I am aware, not 



