Vol. 5] 



Merriam. — Osteology of Nectosaurus. 



223 



that the dentition differs very considerably in the two forms. It 

 should also be noted, that Nectosaurus is not uncommon in the 

 deeper water deposits just above the base of the upper division 

 of the Hosselkns Limestone, where Thalattosaurus has not yet 

 been recognized. 



The vertebrae and ribs agree in structure with what has pre- 

 viously been described for this genus and for Thalattosaurus, ex- 

 cepting that specimens have been obtained showing a distinct 

 bicipital rib articulation in addition to those with but one artic- 

 ulation. The presumption is that the situation here is similar to 

 that in some other rhynchocephalians, and in most Triassic 

 ichthyosaurs, the anterior ribs being double-headed, while single 

 articulation appears in the dorsal region. 



Transmitted September 3, 1907. 

 Issued May 13, 1908. 



