Occasional Papers of the Museum of Zoology 21 



ties rather shallow, just obscuring dorsal scars ; anterior muscle 

 scars deeply impressed but quite smooth; posterior scars 

 well marked but not impressed ; pallial line deep, crenate ; nacre 

 white to reddish violet, almost scarlet, sometimes with a cop- 

 pery tinge, thickened anteriad, iridescent posteriad ; edge of 

 inside of shell, due to obliquity of prismatic layer, with white 

 to rust-colored border 2 to 4 mm. wide. 



Measurements 



.The larger shells of this species seem to be markedly dimor- 

 phic ; those that appear to be the old males somewhat resemble 

 L. fiinbriata Frierson (1907), while those taken for old females 

 have the slightly hooked beaks and the humped posterior dorsal 

 margin of U. discus Lea (1838). On account of this dimor- 

 phism and the resemblance of the two sexes (?) to these two 

 forms, I think it is probable that Unio discus (more normal 

 development U. panacocnsis von d. Busch) is largely based on 

 old female specimens which have reached, in the quieter water 

 of the large river near Tampico, their completely distinctive 

 form, while L. Umbriata Frierson, also from the Panuco River 



