278 
vmomDM. 
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. 
