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. 
Mi&ASUREMENTS 
4 Y ino} pea 
Ss 2S re o te 
a 2G De 
Seoest ee 
ye A 
A. disca (Lea) 13256103 27 (type, Lea, 1838) 
135 Gy/ 24 (Simpson, 1914) 
A. disca fimbriata(Fr.) 80 59 31 (Frierson, 1907) 
Fig. 1 46 52(24) 30(14) 
Fig. 2 6r 56(34) 34(21) 
Fig. 48 (right valve) 97 65(63) 41(40) (female?) 
Fig. 49 105 57(60) 34(36) (type; male?) 
Fig. 50 (left valve) TO25 571050) oi (32) Gnale®) 
Extremes (11 
adults) 88.5-100.5 55-05 31-45 
The larger shells of this species seem to be markedly dimor- 
phic; those that appear to be the old males somewhat resemble 
L. fimbriata 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. panacoensis 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. fimbriata Frierson, also from the Panuco River 
