BEAKS OF UNIONID^E 177 



sion, when present, was located as described above. In 

 U. pressus the depression may run either to the middle of 

 the ventral margin or to a point immediately in front of the 

 posterior ventral angle. 



The beaks of all the specimens of this species which have 

 come under my observation are more or less eroded, but in 

 almost all cases the characters described above can be 

 plainly traced. 



Unio luteolus, Lam. 



Fig. 3 



Beaks with seven to ten undulations, each with a promi- 

 nent irregular sinus on the ventral side about the middle. 

 The undulations lack converging lines and the dorsal areas 

 lack radiating lines. The undulations are rounded and con- 

 tinuous in front, deflected at the sinus in the direction of the 

 posterior ventral angle of the shell. Posteriorly, the later 

 undulations are wavering and interrupted ancl terminate 

 abruptly at the ridge running from the beak to the posterior 

 ventral angle. The last two or three undulations are very 

 weak anteriorly. 



In this species the number of undulations is variable. As 

 a rule the later undulations are more or less interrupted and 

 wavering and the sinus weak, but the earlier undulations 

 are regular and continuous and have the sinus prominent. 

 The undulations of a specimen from Rockford, 111., are so 

 broken and wavering as to give the surface of the young 

 shell a granulated appearance. A great difference in rela- 

 tive coarseness of the undulations may be observed in 

 different specimens. In some they are more numerous, 

 very fine and placed close to each other, giving a rippled 

 appearance. In others they are fewer in number, coarse, 

 and rather widely separated. The beaks of this species are 

 usually eroded but many perfect specimens can be easily 

 obtained. U. luteolus in beak and other characters is closely 

 *3 



