PLATE XXVII 
Unio stewardi White. Jurassic. 
Fic. 1. Left side view; restored from broken specimens. 
Fic. 2. Left side view of a younger specimen. 
Unio nucalis Meek and Hayden. Jurassic. 
Fic. 3. Left side view. 
Fic. 4. Dorsal view of the same specimen. 
Unio mecropisthus White. Jurassic. 
Fic. 5. Left side view. 
Unio iridoides White. Jurassic. 
Fic. 6. Right side view. 
Margaritana nebrascensis Meek. Dakota group of the Cretaceous series. 
Fic. 7. Left side view. 
The species, U. stewardi resembles a common type of Unio in the Missis- 
sippi fauna. 
The figured specimen of U. macropisthus is the only one discovered. Its 
posterior breadth probably indicates that it is a female. 
The species, U. iridoides, closely resembles U. iris, of the Mississippi fauna. 
Margaritana nebrascensis is much more inflated, especially in the umbonal 
region than is any known living species of that genus; but it is known to 
possess the hinge structure of Margaritana. 
