PRECAUDAL VERTEBRA OF ICHTHYOSAURUS AUSTRALIS — ETHERIDGE. 67 



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 three-quarters of an 

 inch wide, the joint faces of the neurapophysial surfaces rather 

 triangular on very strongly raised fore and aft synchondrosial 

 articular elevations, the space between these and the diapophysial 

 tubercles is roughly three inches, the latter having descended in 

 close contiguity to the parapophysial tubercles. It is clear, there- 

 fore, that this vertebra from the wide disassociation of the neura- 

 and diapophyses occupied a position in the 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 para- 

 pophyses form a pair of separate tubercles on each side near the 

 anterior end of the centrum. 



The diapophyses are set further in from the anterior articular 

 edge than the parapophyses ; these 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 interstitial surface 

 being deep and groove like. The haemal surface is quite plain. 



The posterior concave articular surface is infilled with matrix, 

 affording a complete cast of the next succeeding anterior cup, and 

 even retaining a portion of the osseous tissue of the latter adhering 

 to it. This tissue throughout the centrum is well preserved and 

 dense. 



The specimen is certainly of the Campylodont group of Ichthyo- 

 sauri, and occupies an intermediate position in outline between 

 an "early posterior dorsal" and a "late posterior dorsal" vertebra 

 of /. trigomis, Owen.* 



The largest Ichthyosauri attained a length of from thirty to 

 forty feet, and the present meridianal species was in no way 

 inferior to its gigantic fellows of the European Secondary seas. 

 If we apply a similar method of arriving at the comparative size 

 of an Ichthyosaurus as that adopted by Prof. Owen, that the 

 jaw was " thirteen times the length of the vertical diameter of 

 an abdominal or anterior caudal centrum," we see in the present 

 fossil the representative of an animal possessing a jaw a little 

 over five feet in length — thus 13 x 5' = 65" = 5' 5" long. Prof. 

 McCoy computed! the remains of his type specimen to represent 



* Lydekker — Brit. Mus. Cat. Foss. Reptilia and Amphibia, 1889, pt. 2, 

 p. 26, figs. 13 and 14. 



t Trans. Roy. Soc. Viet , ix., 2, 1869, p. 77. 



