144 Mr. K. Etheridge, Juu., on a 



having the articular surfaces of the first and last well dis- 

 played. The positions of the diapophysial and ])leurapo- 

 physial articular surfaces leads to the belief that the vertebra 

 is one of the middle trunk series. It is subcircular in outline, 

 slightly narrowed and contracted neurally. Measured across 

 the articular surfaces from the neural to the hgemal margins 

 the diameter is exactly 5 inches, and in a transverse direction 

 from diapophysis to diapophysis it is an eighth of an inch 

 short of a similar measurement. Longitudinally measured 

 between the dia- and plcurapophysial tubercles tiie centrum is 

 exactly 2 inches, but on the haemal surface it is a quarter of 

 an inch more. 



The concave terminal articular surface visible is deep, ter- 

 minating in a central fossa, the extent of the concavity being- 

 well exemplified by the matrix cast of the anterior cavity of 

 the succeeding vertebrae at the posterior end of this specimen. 

 This mass of matrix represents the "elastic capsule" that 

 intervened between the vertebra, and retains on its surface 

 portions of the osseous tissue of the succeeding centrum. 

 The periphery or immediate articular rim at each end is 

 narrow, the surface thence sloping rapidly inwards, but the 

 edges of the rims project slightly outwards, thus rendering 

 the longitudinal or lateral surfaces of the centrum somewhat 

 concave. The depth of the concavities is an inch, or 

 perhaps a little more, and a longitudinal section of the centrum 

 would be, in consequence, of a strongly hour-glass-shaped 

 outline. The floor of the myelonal canal is f of an inch wide, 

 the joint-faces of the neurapophy.<ial surfaces rather triangular 

 on very strongly raised fore and aft synchondrosial articular 

 elevations ; the space between these and the diapophysial 

 tubercles is roughly 3 inches, the latter having descended in 

 close contiguity to the parapophysial tubercles. It is clear, 

 therefore, that this vertebra, from the wide disassociation of 

 the ncura- and diapophyses, occupied a position in tlie column 

 certainly more than one third of the trunk from the head, and, 

 according to Owen's measurements, was near about the forty 

 to forty-fifth vertebra, for in this region in Ichthyosaurus the 

 dia- and parapophyses form a pair of separate tubercles on 

 each side near the anterior end of the centrum. 



The diapophyses are set further in from the antei ior articular 

 edge than the para])0physes ; tiiese are close to the latter, but 

 are not connected with it by a " neck." Both are represented 

 by large and strong rounded tubercles, separated from one 

 another by an interval of two-eighths of an inch, this inter- 

 stitial surface being deep and groove-like. The haemal sur- 

 face is quite plain. 



