1916] Wiman: Marine Triassic Reptile Fauna of Spitsbergen 



65 



The Posidonomya slates, as well as the following Daonella slates, with 

 the upper saurian horizon, belong to the Musehelkalk. Above these 

 bituminous slates again come arenaceous beds which, for the greater 

 part at least, ought to represent the Keuper. Within these lies the 

 new vertebrate horizon. 



The age relations of the better known Triassic ichthyosaurians 

 thus becomes the following : To the Middle Triassic belong the finds 

 in Germany, Nevada, and Spitzbergen, and to the Upper Triassic those 

 in California and at Besano. 



MIXOSAUEUS AND PHALAKODON 



The genus Mixosaurus was erected by G. Bam* 12 in 1887, and was 

 founded on the Lombardy species M. cornalianus Bass., which now, by 

 the researches of Repossi 18 and myself, 21 is pretty well known. In 

 1891 E. Fraas r ' incorporated with this genus, under the name Ich- 

 thyosaurus atavus, the species originally described by Quenstedt. 17 

 This procedure was accounted for by I. atavus having elongated fore- 

 arm bones, sparse and dimorphous teeth, etc., characters which one 

 now knows are peculiar to Triassic ichthyosaurians in general, and 

 therefore cannot be used as generic characters. For the same reason 

 Dames 3 took this view. Judging by the little hitherto known of 

 I. atavus, it cannot belong to the genus Mixosaurus. 



For still less important reasons Dames referred to the genus Mixo- 

 saurus the smaller of the only two species from the Spitzbergen 

 Triassic. This, however, has turned out more successful, and new 

 material having been added since 1909, I did not hesitate, in 1910, to 

 class Mixosaurus nordenskidldi with this genus. In this view I have 

 been further strengthened since I succeeded in obtaining material of 

 the Besano species and studied Repossi 's original specimens in Milan. 



In 1910 a work by Merriam 12 was published in which a new species, 

 Phalarodon f raasi, was described. It then appeared that I had prob- 

 ably made a mistake, in so far as I had also classed the Phalarodon- 

 like jaw fragments with Mixosaurus nordenskidldi (1910, pi. 5, figs. 

 10, 12, and 13). In so doing I conceived the possibility of a difference 

 in sex of such a nature that the less pronounced dimorphous teeth 

 might represent, for example, young females and the more developed 

 dimorphous teeth old males. My figure 10 shows a transition, on the 

 one hand, from those which I still wish to class with Mixosaurus 

 nordenskidldi (figs. 9 and 11) and the perfectly Phalarodon-like ones 

 in figures 12 and 13. 



