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University of California Publications in Geology [Vol. 10 



In his work of 1911 Merriam 13 called attention to the similarity 

 between Phalarodon and certain of my specimens, and I now consider 

 it probable that the jaw fragments (pi. 5, figs. 12 and 13) at least, 

 are to be connected with Phalarodon, which is a special type. After 

 having seen comprehensive material of Mixosaurus cornalianus, I have 

 found that the appearance of the teeth is very constant in different 

 specimens, and from investigations by Dollo 4 of the mosasaurian 

 Globidens fraasi, in which also Phalarodon is treated, it has also be- 

 come probable that the posterior teeth of Phalarodon indicate a very 

 diverging diet. As, however, there is in the grade of dimorphism of 

 the teeth a very even transition from M. cornalianus over M. norden- 

 skioldi and to the pronounced Phalarodon type, one ought perhaps to 

 await a more complete knowledge of the crania of the Mixosaurus 

 species, and new discoveries of remains of PJtalarodon, of which 

 hitherto only cranium parts are known, before one decides where the 

 boundary between Mixosaurus and Phalarodon should be fixed, and 

 whether the difference really is so great that it warrants the setting 

 up of a new genus. For the present I will, therefore, keep M. corna- 

 lianus and .1/. nordenskioldi together within the same genus. To it 

 belongs, with a certain degree of possibility, also Cymbospondylus? 

 nutans Merriam from Nevada. The anterior limb indicates, as will 

 be seen by the accompanying figures (figs. 1 and 2), great similarity, 

 especially to the species from Spitsbergen. The piece of the vertebral 

 column, which Merriam classes with this species, does not contradict 

 this supposition. It is perhaps not excluded that Phalarodon fraasi 

 is the cranium of this species, for it originates from the same horizon. 



The pelvic arch of M. nordenskioldi is, as I have previously shown, 

 almost identical with that of M. cornalianus, which has also a foramen 

 pubicum. 



As Merriam has rightly observed from my figure of the vertebral 

 column, a portion of the tail is missing, but it is seen from loose 

 vertebrae that the tail has not narrowed off suddenly, as with the 

 w hales, but very slowly, as with other ichthyosaurians. The part of 

 the tail that lay behind the caudal fin must therefore have been very 

 long. Merriam also conceives of the possibility of my having placed 

 the pelvis too far back. This I do not consider probable, for I am 

 very sure about the position of the pelvis. The difficulties that Mer- 

 riam finds in the, with all this connected, placing of the caudal fin 

 disappears if one takes another view of the origin of the caudal fin 

 than Merriam. I have imagined that the caudal fin is a dorsal fin, 



