APRACTOCLEIDUS TERETIPES. 



613 



trace of ever having been fused (text-fig. 6, A). They have large, rough convex 

 heads (/>.), which are accommodated by corresponding concavities in the centra. The 

 free cervical ribs are much less compressed from above downward (cf. text-figs. 2, c.r., 

 and 6, A), and about the twenty-seventh a section through the neck of the rib is an 

 equilateral triangle. It is noteworthy that the neural arches of the posterior 

 cervical vertebrae are closely fused to the centra, the sutures being even less visible 

 than in the anterior cervicals, so that the freedom of the ribs is not matched by any 

 corresponding looseness in the neural arch of the same vertebra. The freedom and 

 lack of breadth of the ribs in the posterior of the neck must have served to mitigate 





Text-Fig. 3. 



Pectoral vertebrae of Apractocleidus teretipcs : from left side. (Type specimen 

 V. 1091, J nat. size.) a.z., anterior zygapophysis ; n.s., neural spine; 

 p.z., posterior zygapophysis ; r.f., rib facet. 



the excessive rigidity which, as insisted on by Williston (1913), must have existed 

 in the thick portion of the neck of Plesiosaurs. 



Pectoral vertebrae (text-fig. 3).— The sutures between neural arch and centrum 

 are so indistinct in posterior cervical and pectoral regions that nothing definite can 

 be deduced from them. The rib facets begin to rise on the centra about the 

 twenty-fifth cervical vertebra, and after the twenty-seventh a sudden change occurs— 

 instead of the rib articulating in a rough depression in the side of the centrum, a 

 short process with smooth termination receives the rib, and this process is situated 

 higher up on the centrum. The next two vertebrae are very similar, but show the 

 process steadily ascending on the side of the centrum. The smooth terminations of 

 the processes (r.f.) of the pectoral vertebrae show slight sinuosities on their other- 

 wise fiat surfaces. In the next vertebra (the first dorsal) the transverse process 

 is borne wholly on the neural arch, is considerably longer, and has a decidedly 

 convex oval termination whose anterior dorsal margin is reflexed. In this region 



