BEAKS OF UNIONID^E 1 79 



decide whether these elevations are or are not of the same 

 nature as the undulations. For this reason, and because of 

 the small area occupied by the undulations, which, to me, 

 appears to be abnormally small, I fear that the description 

 will not answer for any specimens other than the one from 

 which the description is taken. 



Unio nasutus, Say 



Fig- 5 



Beaks with five or six undulations which are rounded on 

 the summit and placed nearly parallel to the hinge-line. 

 Each undulation has a prominent sinus on the ventral side 

 back of the middle. Posteriorly, the first and second undu- 

 lations have very faint converging lines ; remaining undu- 

 lations lacking converging lines and posterior dorsal area 

 lacking radiating lines. Anteriorly, the undulations have no 

 converging lines of the usual type but the undulations are 

 rounded and continue up the anterior dorsal area nearly to 

 the hinge-line. Posteriorly, the undulations, except the 

 first and second, terminate at the ridge traversing the shell 

 from the beak to the posterior ventral angle. On the pos- 

 terior area the concentric striae are very numerous and 

 much more prominent than elsewhere. In most of the very 

 young specimens there may be seen two slight depressions 

 of the shell along the posterior dorsal area — these depres- 

 sions are continued throughout the growth of the shell. 



The undulations of the beaks of U. nasutus resemble, in 

 some measure, those of U. radiatus, Lam., while a much 

 closer resemblance exists between the former and U. luteotus, 

 but the outline of the young nasutus differs so much from 

 the outlines of the other two that the shell cannot be mis- 

 taken for either of them. Notwithstanding the similarity of 

 beak characters existing between U. nasutus and U. luteolus 

 and U. radiatus the great differences in the other characters 

 of the shells do not permit the supposition that the former 

 is closely related to either of the latter. 



