University of California Publications. 



[Geology 



the tendency of the neural arches to fuse with the centra. A 

 double articulation of the ribs such as is seen in some of the 

 Nectosaurus specimens has not thus far been known in Thalatto- 

 saurus. The rib articulation is however only slightly different 

 in the two genera. Though there is no absolute division of 

 the lateral apophyses of the vertebrae in Thalattosaurus, the 

 apophyses of the dorsal vertebrae are known to be strongly con- 

 stricted. The position of the rib head with reference to the 

 centrum and the upper arch is the same in the two genera. 



A number of ribs showing well preserved articular faces have 

 been obtained and in nearly all of these the head and tubercle 

 are distinctly separated by a deep notch (pi. 18, fig. 3). 



Conclusions. 



The characters of the elements of the skull and of the dentition 

 of Nectosaurus are essentially of the thalattosaurian type. The 

 nature of the attachment of the teeth is more clearly shown 

 than it has been previously, and indicates that teeth may be set 

 in deep, distinct pits without apparent anchylosis with the jaw, 

 or their bases may be fused with the bone. 



The limb elements obtained are all of a more elongated or 

 more slender type than is seen in the typical Thalattosaurus. 

 Those propodial elements which are well enough preserved to 

 show the outlines of the bones are even more slender than the 

 humerus of the small T. shastensis. Such material as is available 

 seems to indicate that the epipodials and possibly the podials 3 were 

 little more specialized than in typical shore forms. The suggestion 

 made by the writer in a previous publication 4 that Nectosaurus 

 may have been a shore dweller to a greater extent than Thalat- 

 tosaurus, seems to be supported not only by the less distinctly 

 specialized or adapted limbs, but also by the fact that the remains 

 are most common in deposits containing considerable quantities 

 of comminuted shells, which would naturally occur near the 

 shore. The suggestion that Nectosaurus represents the shore- 

 dwelling young of Thalattosaurus seems to be met by the fact 



s The term podials as used here is intended to include metapodials and 

 phalangeal elements. 



* Merriam, J. C, Op. cit., Vol. 5, p. 23. 



