390 



University of California Publications. [Geology 



tition of the ancestor might possibly be expressed in any one of 

 several tooth forms. The general occurrence of a thecodont 

 tooth insertion in early ichthyosanrs suggests that Phalarodon 

 is probably not less primitive than the other forms. As to 

 whether the ancestral ichthyosaur possessed a highly differen- 

 tiated dentition is somewhat doubtful, but the differentiation of 

 the dentition in the European Mixosaurus, in Phalarodon ( f) 

 atavus, and in the American Phalarodon suggests that differen- 

 tiation may be a primitive character. A reexamination of the 

 dentition of Cymoospondylus has not as yet shown any evidence 

 of marked differentiation. 



Issued January 21, 1910. 



