* This species was first named heterodon by my father, in con- 

 versation with Mr. Mason, long before the description was 

 published or even written. Mr. Mason communicated the 

 name as well as the species to Mr. Lea, who gives his friend 

 credit neither for the name nor for discovery of the fact that the 

 lateral tooth is double in the right valve, as injustice he should 

 have done. I make these remarks, because Mr. Lea insists 

 that U. plicatus, Say, must be credited to Leseuer; and, if so, the 

 species in question must be called U. heterodon, of Mason, as the 

 circumstances under which these two species were described are 

 parallel. 



