Vol. 5] 



Merriam. — Osteology of Nectosaurus. 



221 



The upper arches are considerably higher than the centra. 

 In specimen 10774 (pi. 18, fig. 6), showing a double headed ar- 

 ticulation with the rib, the spine is considerably flattened lateral- 

 ly, but is not particularly broad anteroposterior^' . In specimen 

 10779 (pi. 18, fig. 5) with no lateral apophyses, and evidently 

 representing the caudal region, the spine is high and verj slender 

 with almost no lateral compression. Zygapophyses are distinctly 

 shown even in the caudal vertebra (No. 10779), where they are 

 very prominent. In most cases the vertical element in the posi- 

 tion of the faces of articulation seems to be much greater than the 

 horizontal. In one specimen (No. 10787) the faces seem to have 

 come nearer to a horizontal plane, but they are somewhat damaged 

 and it is not possible to be absolutely certain of this determination. 



The lateral apophyses in articulation with the ribs are single 

 on some specimens and widely divided on others. On specimen 

 No. 10627 (pi. 18, fig. 4) the large and prominent apophysis is 

 nearly as high as the centrum, but is clearly undivided on its 

 finely exposed articular face. A nearly obliterated line of divi- 

 sion between the upper arch and the centrum appears to cross the 

 apophysis some distance above the middle, as in the anterior ver- 

 tebrae of Thalattosaurus. In specimen No. 10774 there are two 

 distinctly separated lateral apophyses. The parapophysis is low 

 down on the anterior margin of the centrum. The articulation 

 of the tubercle seems to be about equally divided between the 

 upper margin of the centrum and the base of the lower arch. 



On specimen No. 10787 there is evidence of separation of a 

 distinct inferior apophysis. The zygapophyseal faces are rela- 

 tively large on this specimen and have a more nearly horizontal 

 position than in most of the others. In this specimen the neural 

 arch resembles considerably the anterior dorsal or cervical arch 

 described with the type specimen of Thalattosaurus sliastensis. 

 If this is the case the double rib articulation might be suspected 

 to occur in the anterior region of the vertebral column, the un- 

 divided apophyses being present on middle or posterior dorsals. 



The structure of the vertebrae here referred to Nectosaurus is 

 in general much like that in the genus Thalattosaurus. The two 

 genera resemble each other and differ from the Ichthyosauria in 

 the relatively slight degree of biconcavity of the centra, and in 



