MEHHIA.M: TRIASSIC ICHTHYOSAl'RI A. 



49 



Fig. 59. Toretocnemus calif ornicus Mer- 

 riam. Vertebrae and rib from middle, 

 dorsal region, natural size. 



retain nearly the same size from the axis to the twelfth centrum. On the 

 thirteenth the diapophyses are suddenly elongated, and there is no parapo- 

 physis on one side, though a rudiment is present on the other side. The 

 diapophyses have a peculiar form on this cen- 

 trum, the lower ends being turned backward in- 

 stead of forward. 



In Deljiliiiioxduru* the same relation obtains 

 as in Shwtasaurus, and in Men-nutria we know 

 only that the middle dorsal ribs are single- 

 headed. 



In Torctocucinnti (fig. 59) the middle and 

 posterior dorsal ribs have deeply notched or 

 forked heads articulating on distinctly separated 

 diapophyses and parapophyses. The vertebrae 

 and ribs of the anterior region of the column are 

 unknown. 



In the European Mixosaurus the 

 rib articulations are said to be unicip- 

 ital excepting a possible faint division 

 in the posterior dorsal and anterior 

 caudal region (figs. 60 and 60?;). 



In Mixosaurus (t) atavus of the 

 European Muschelkalk the only ante- 

 rior dorsal vertebra known (fig. 61) 

 seems to have a double-headed rib 

 articulation. Several posterior dor- 

 sals from the collections at Stuttgart 

 (fig. 63) and Zurich have distinctly 

 separated diapophyses and parapo- 

 physes low down on the sides of the centra. Other centra (figs. 64a and 64&) 

 not far from the region of the pelvis have a single, large, lateral apophysis in 

 which there may be some indication of narrowing near the middle. These last 

 centra show no traces of facets for intercentra, and if located behind the pel- 

 vis could not be situated very far from it. 



There are no vertebral centra available which certainly represent the middle 

 dorsal region of 3/.(?) atavus unless some of those which have been referred 

 to the anterior and posterior portions belong here. A peculiar centrum figured 

 by Fraas (fig. 62) shows a cross-section similar to that of a middle dorsal, and 

 possesses a small but prominent apophysis a little above the middle of each 

 side. The writer has not seen so small an apophysis for unicipital articula- 

 tion of middle dorsal ribs in any ichthyosaurs, but apophyses of this size, form, 



Figs. 60a and 606. Mixosaurus cornalianus (Bassani). 

 Ribs natural size. (After Repossi.) Fig. 60a, dorsal 

 rib; fig. 606, posterior or anterior dorsal rib. 



