1911] Merriam: Saurian Fauna of Spitsbergen. 



323 



spines immediately behind a break in the specimen, which sug- 

 gests, as a bare possibility, the loss of a portion of the caudal 

 series at this point. It is also worthy of note that the original 

 position of the pelvis is very difficult to determine, and it may 

 easily have been situated considerably farther forward than the 

 point in the column below which the ilium now rests. A modi- 

 fication in one or both of these particulars might change the 

 relative position of the upper lobe of the caudal fin. It would 

 seem to the writer that a propeller constructed as represented 

 in this form would be far from the most efficient type. It 

 would also represent a- distinct deviation from what appears 

 to be the natural line of evolution of the typical ichthyosaur 

 out of a Palaeohatteria-like shore form. In an earlier publi- 

 cation 9 the writer has suggested that the probable course of 

 evolution of the ichthyosaurian caudal fin has been through a 

 series of forms in which the tail fin was first distinctly adapted 

 for service as a propeller by increase in height some distance 

 anterior to the tip through elongation of the neural arches. 

 This was possibly accompanied by a slight droop of the extreme 

 posterior region. Through the stages of Cymbospondylus{t). 

 natans and Mixosaurus cornalianus a superior lobe developed 

 over the elongated spines. Once developed, this lobe remained 

 and increased in importance. The support of the neural spines 

 came to be unnecessary and their arches were much reduced in 

 the later stages of evolution of the group. 



The rib articulation is not clearly shown for the whole verte- 

 bral series, and is unfortunately not known in the cervical and 

 anterior dorsal regions. Some of the vertebrae bear widely sep- 

 arated diapophyses and parapophyses as in Toretocnemus from 

 the West American Triassic. These centra are from the middle 

 or posterior dorsal region. 



PESSOSAUBUS POLARIS (Hulke) 

 A number of specimens representing a large saurian from 

 the upper bed are included by Wiman in Hulke 's species 

 Ichthyosaurus polaris, which is, however, separated as a distinct 



» Merriam, J. C, Triassic Iehthyosauria, Mem. Univ. Calif., vol. 1, p. 

 40. 1908. 



