ART. 1 NEW LAND AND FRESH-WATER MOLLUSKS MARSHALL 3 



Anterior margin regularly curved, rounding into the ventral margin, 

 which is slightly curved but subparallel to the dorsal margin. Beak 

 set far forward, about 10 mm. behind the extreme anterior end and 

 56 mm. in front of the extreme posterior end. A distinct lunule 

 under the beak. Beak ornamented with a number of concentric, 

 waving undulations, the main portion of each subparallel to the 

 dorsal margin and strongest on the posterior ridge. On the pos- 

 terior area a fine, nearly straight, threadlike line runs from each 

 undulation in the direction of the beak. Anteriorly, irregular, 

 nearly obsolete lines curve from the undulations to the beak. Pos- 

 terior ridge high; anterior ridge not differentiated from the general 

 surface of the shell. Concentric sculpture consisting of many deeply 

 impressed lines of growth, with indications of fine concentric striae 

 between them. Posterior half of the shell with several very promi- 

 nent plications, set obliquely across the surface, parallel to the 

 posterior ridge. In front of these are indications of several other 

 plications which have become obsolete or which did not develop. On 

 the posterior area of the young shell are several fine lines running 

 from the ridge to the posterior margin. These may indicate the 

 flutings found on the posterior area of many plicate naiiades, but 

 which can not be seen in this specimen because part of the later 

 shell is broken away. 



The type (Cat. No. 370999, U.S.N.M.) measures: Length, 66 mm.; 

 height, 47 mm.; diameter, if both valves were present, would be 

 about 36 mm. 



As pointed out in the description of the genus Pliconaias, this shell 

 is very closely related to the plicate North American naiades and 

 may be the ancestor of some or all of them. The sculpture of the 

 beak is such that the loss of a few undulations would convert it to 

 the style found in Amhlema, while an increase in their number and 

 irregularity might produce the kind found in Megalonaias and 

 Pledomerus. 



The species is named in honor of Willis P. Popenoe, of the United 

 States Geological Survey. 



EONAIAS, new genus 



Fresh-water mussels of the family Unionidae, having the beaks 

 with numerous V-shaped loops, which are nearly regularly spaced, 

 and which " nest " into each other, the V's pointing toward the ventral 

 margin. Posterior area with fine riblets running from the posterior 

 ridge to the margin. Type Eonaias reynosenica, new species, described 

 below. 



The nearest relative seems to be Quadrula petrina Gould, a pearly 

 mussel now living in Texan waters, and which has a somewhat similar 

 form and beak sculpture. Eonaias shows also some relation to 



