MKi;l;lAM TIIK THALATTdSAriMA 19 



The surface of artirulation is coiisideralily iiarrowcil in the iiiiddU' Imt the apoi)h- 

 vsis is not seen to divide into distinet di- and i)ar- apopliyses. In the dorsal 

 vertchrae examined, the uppermost portion of tlic ril) head lias heen in articu- 

 lation with the hase of the neural arch. 



In the caudals of Thaluttofianru.i (IM. vii, i\<j,. '■',) the centra are somewhat 

 angular in cross-section, though not greatly flattened laterally. The uppei- arches 

 are very slender and relatively thick. Large and [ironunent zygapophyses are 

 present. The faces are sharply separated and almost vertical in position. A slight 

 elevation of the neural spines may indicate a little l)roadening of the tail. The 

 structure of the vertebrae seems to indicate that the tail of T ha I aitosaurus was not 

 a highly specialized sculling organ as in the Ichthyosauria and the Thalatto- 

 suchia. The propelling function may have been performed largely by the limbs 

 as in the Plesiosauria. Evidence pointing in this direction is furnished by the 

 bones of the pelvic arch, which are relatively large and robust for a swimming 

 animal of this size. 



Ril.s. 



I^i.ATu VI r. Figs. 6 and 7. 



True Riha. — Many fragments of ribs are present in the type specimen. In 

 these the shaft is apparently about as heavy as in 'the lehthyosaurs. The 

 middle of the shaft is grooveil on one side, the distal end is round in cross- 

 section. In the single specimen in which the liead is preserved but one articu- 

 lar surface is seen. In a specimen of 2. sJiasfensis showing several rib fragments 

 associated with the bones of tiie head and the pectoral arch (PI. vii, fig. 7) the 

 shaft is ratliei- heavv, is groove(l anteriorly, and there is a single wide articular 

 face as in Shasfasaurus. 



In the dorsal region the rib heads have articulated largely on the broad 

 ai)ophyses on the centra of the \-ertel)ra, but the uppermost portion rested also 

 in })art against the base of the upper arch. 



Alxloiiiiital Ribs. — ( »n one specimen (No. 9120) numerous abdonunal ribs 

 are present. They are long, and compared with the dorsal ribs are exceedingly 

 delicate and slender. It has not been possible to distinguish between median 

 and lateral pieces. 



In the tvpe specimen the lower j)ortion of the skeleton with the abdominal 

 ribs had been largely destroyed, Imt a single fragment ap[)arently representing 

 an alidominal rib is seen between the distal ends of the dorsal ribs. 



