Vol. 5] 



Merriam. — A Prim itivc I eh th yosa urian . 



389 



The dentition most closely resembles that of Mixosaurus(?) 

 atavus (Quenstedt) as described by Fraas. 3 The specimens 

 available for examination by Fraas showed unfortunately a very 

 few teeth, but careful observations have indicated that, as in 

 the American Phalarodon, the dentition was differentiated; while 

 the roots were coarsely folded, were set in partly or entirely 

 separated alveoli, and possessed little or no cement covering. 



The amount of differentiation in the European M. (?) atavus 

 is unfortunately not clearly shown. It is, however, noted that 

 the most posterior teeth have low, domed, laterally-compressed 

 crowns, while the crowns of the teeth immediately anterior are 

 more slender and less flattened laterally. 



The dental characters of M. (?) atavus are so close to those 

 of Phalarodon as to suggest very strongly the generic identity 

 of the two. On the other hand the dentition of both forms 

 appears to differ considerably from that of the Italian Mi.ro- 

 saurus as described by Repossi. 4 As has been previously sug- 

 gested by the writer 5 the vertebrae of the M. (?) atavus forms 

 seem to differ in some particulars from those of the typical 

 Mixosaurus. It appears reasonable to give the M.(?) atavus 

 forms a tentative position in the genus Phalarodon until further 

 evidence can be obtained regarding the structure of both the 

 American and the European material. 



In general skull characters Plialarodon seems to represent a 

 member of the Ichthyosauridae more primitive than any form 

 heretofore described. The relatively large exposed area of the 

 frontals, and the relatively small nasals, approach more nearly 

 the form and the relative size of these elements as seen in the 

 early Reptilia generally, than we find them in later ichthyosaurs, 

 or even in the Middle Triassic Gymbospondylus. The maxil- 

 laries seem also to be comparatively large. It is hardly possible 

 to state definitely whether or not the dentition is primitive, as 

 primitiveness in the sense of closer corresponden.ee to the den- 



s Fraas, E., Ichthyosaurier der Siiddeutschen Trias-und-Jiira-Ablagernn- 

 gen, 1891, S. 38, and Taf . 3, fig. 2 and 3. 



4 Eepossi, E., Mixosauro degli strati Triasici di Besano in Lombardia. 

 Atti della Soc. Ital. di Sc. Nat., vol. 41, tav. 8. 



5 Merriam, J. C, Am. Jour. Sc., vol. 19, p. 30, Jan., 1905. 



