28 



MEMOIRS OF THE UNIVERSITY OF CALIFORNIA. 



dentition of the posterior half of the jaws is known. In Mcn-itnnia and in 

 Sli(ixl<in(iun<n the teeth, as far as known, are set in open grooves. Imperfect 

 alveoli may, however, have been present in some of the regions of the jaw which 

 have not yet been examined. 



The characters seen in the American Triassic forms are in strong contrast 

 to the nature of the dentition of MixowiHni* (figs. 13 and 14), in which the 

 teeth show marked differentiation and are set in rather widely spaced distinct 

 pits. 



Fig. 13. Mixosaurus cornalianus (Bassani). Up;>er jaw :in<l dentition, natural size. (Adapted 

 from Repossi.) 



Fig. 14. Mixosaurus cornalianus (Bassani). Lo'.ver jaw and dentition, natural size. (Adapted 

 from Repossi.) 



As has been shown by Fraas (1891, p. 38) the dentition of Mixosaurus ( ?) 

 atavus (figs. 15 and 16) is characterized by extreme differentiation of the teeth, 

 by their firm insertion in slightly separated alveoli, by the coarse folds of the 

 basal portion of the dentine cone, and by the extreme reduction of the cement. 



16 



Fig. 15. Mixosaurus ( ?) atavus (Quenstedt). Anterior end of lower jaw with dentition, natural 



size. (After Quenstedt.) 



Fig. 16. Mixosaurus ( ?) atavus (Quenstedt). Two forms of teeth from the lower jaw, natural 



size. (After Fraas.) 



The peculiar characters of the dentition of Mixosaurus, occurring as they 

 have with other features which undoubtedly indicate a primitive stage of evo- 

 lution, have suggested that possibly the dentition is essentially primitive, and 

 that an original differentiated stage of a thecodont dentition which developed 

 in the ancestors of the ichthyosaurs has been lost in the adaptation of the later 

 forms, but is retained in the primitive Mixosaurus. If, as appears to be the 



