124 Bulletin American Museum of Natural History. [Vol. XXXIV, 
rather reminding one of U. littoralis; no beak-sculpture visible. 699 
includes a broader (wider) specimen. 18695 is a great lot of haydeni from 
near Fort Bridger; it keeps the characters well, except that some are 
wider. Striation quite coarse.” 
The sketches I made show that the species is unusually flattened, or 
when broader has the posterior end flattened, so that the outline, seen from 
above, is unlike that of U. eomargaron. The straight (oblique) posterior 
slope is also quite unlike that of U. eomargaron. 'The straight lower edge 
separates the new species from U. rectordes and U. shoshonensis. Unio 
shoshonensis White, at least as represented in the National Museum from 
Henry’s Fork, is a quite thick, deep species, with the umbones strongly 
undulate-plicate, with double loop in U. mendaz style, the anterior part of 
loop largest. The dentition is not quite like that of washakiensis. 
Some shoshonensis (det. Marcou, apparently correctly) are from the 
Green River Group, Dry Mts., N. W. Colorado (13223).1. These have the 
shell 4 mm. thick. U. shoshonensis is less compressed posteriorly than 
haydenz, it is in fact quite a ventricose species. 
Unio didymictidis n. sp. 
Shell oblong, the outline almost as in Lampsilis ligamentina, except that the shell 
is not so deep posteriorly, and the lower margin consequently is not oblique; denti- 
Fig. 3. Unio didymictidis n. sp. A, Left valve, from within; B, anterior end, from 
above. 
tion heavy, the posterior pseudocardinal thick and obtuse (evidently somewhat 
worn); umbones not’ at all prominent, too worn to show sculpture; shell thick, 
1 Shells labelled U. clinopisthus White (det. Marcou), from Green BR. Group, near Raven 
Park, Colorado (8876) are not that species, but are probably haydeni, so far as the poor 
material shows. A lot of shells from Henry’s Fork are genuine clinopisthus. 
