STENEOSAURUS. 93 



It is interesting to note that the greater elongation and slenderness of the shall in 

 St. leedsi as compared with that of St. durobrivensis are seen also in the proportions of 

 the vertebral centra of the two species, particularly in those belonging to the cervical 

 and caudal regions. 



The atlas and axis (text-fig. 34) of the Oxford Clay Steneosaurs were described by 

 Hulke* in 188S, and recently by Auerf who has published a more detailed account, 

 with a useful and complete summary of the views that have been put forward as 

 to the homologies of the constituent parts of the atlas-axis complex. Here the 

 interpretation adopted is the same as that employed in the description of the atlas-axis 

 of Veloneustes given above (p. 47) J. 



The atlantal ring is composed, as usual, of the anterior ventral wedge-bone (a.w.l.) 

 and the two supero-lateral pieces, which are produced up into the two halves of the 

 neural arch [at. a.). The surface of the ventral wedge-bone, forming the lower 

 portion of the atlantal cup, is strongly concave ; beneath this it becomes roughened 

 and strongly convex, forming a well-developed ventral prominence. On its lateral 

 surfaces is a pair of short projections, each bearing a triangular articular surface for 

 the first pair of ribs (r l .f.) ; these surfaces look nearly directly backwards. Above the 

 rib-facets the wedge-bone narrows, terminating dorsally on either side in a small facet 

 looking directly upwards, for union with the lower ends of the lateral (neural) pieces 

 (at.a.) ; these form the supero-lateral portions of the atlantal cup. From their union 

 with the upper ends of the anterior wedge-bone they widen out upwards, spreading 

 backwards over the odontoid [od.) (centrum of atlas), with the obliquely truncated 

 anterior upper surface of which they closely unite. The pedicles of the neural arch 

 extend nearly the whole length of the lateral pieces, and are only slightly notched on 

 their posterior side. The arch itself in some cases seems to have been complete above ; 

 in other Crocodiles the sides of the arch do not unite in the mid-dorsal line, there being 

 a separate roofing-piece. Possibly the condition observed in some of the present 

 specimens is due to the fusion of the lateral pieces with the roofing-piece. Posteriorly 

 the arch is produced backwards into a pair of prominent posterior zygapophyses, which 

 usually fuse with the anterior zygapophyses of the axis. 



The odontoid (od.) (centrum of the atlas) is a large massive element : anteriorly it 



* Hulke, " Contributions to the Skeletal Anatomy of the Mesosuchia, based on Eossil Remains from the 

 Clays near Peterborough, in the Collection of A. Leeds, Esq.," Proc. Zool. Soc. 18S8, p. 417. 



t Auer, " Ueber einige Krokodile der Juraformation," Palasontographica, vol. lv. (1909) p. 217. 



t There seems to be little doubt that the atlas and axis, such as have been described in the Sauropterygia 

 and are here found in Steneoscnirus, are really two vertebra; in which a temuospondylous condition has to a 

 great extent persisted ; the ventral wedge-bones being the hypoeentra (the posterior one is not ossified in 

 the Crocodiles), the odontoid and the centrum of the axis the fused pleurocentra, and the neural arches 

 the neurocenfra of such vertebrae as are found in Desmospondylus, Trhnerorliachis, &.c. This was Cope's 

 original view, which has been strongly advocated by Paur (Amer. Nat. 1897, p. 975), and quite recently by 

 Williston (Journal of Geology, vol. xviii. (1910) p. 594). 



