278 UNIONID-E. 



age ; and it has hence been regarded as a species by 

 Michaud and others, under the name of U. Roissyi. 

 But in general, as they enlarge, they become much 

 eroded about the umbones, and the lower margin is 

 more or less drawn in. In this state it is the U. 

 elongatus of Lamarck. I do not know whether the 

 erosion of the beak has anything to do with the con- 

 traction of the lower edge ; but all the oblong regular 

 specimens I have seen have the periostraca of the 

 beaks only slightly worn, while, on the contrary, all 

 those that have the beaks much and extensively 

 eroded belong to the other variety ; and the extent 

 of the inflection generally agrees with the state of 

 the umbo. 



Figure 9. of Plate 2. represents the intermediate 

 state between the two varieties; and figure 10. much 

 more resembles a younger specimen, both in form 

 and colour, than any English Unio I have ever 

 seen. 



3. Unio Retz. (Union.) 



Shell oblong-elongate ; hinge with a short, crested, 

 irregular anterior, and an elongated, laminar 

 posterior, lateral tooth in the right valve, 

 shutting between two similar teeth in the other 

 valve. 



Lives in slow rivers and streams. 



These shells are liable to distortion about the 

 hinge ; the margin becomes thinner and extended ; 

 the shell truncated in front, the lunule enlarged and 

 irregular, and the teeth nearly obliterated, which 

 gives a wedge-shaped appearance to the shell. 



