Lamarck’s Genera of Shells. 309 
almost.like a single tooth, one of them being very large, and 
the other but very slightly prominent. 
One Species. Crassina Danmoniensis*. (Venus Danmoniensis. 
Montague.) 
Shell orbiculo-triangular, browaish yellow, transversely 
wrinkled ; wrinkles parallelly striated, scalariform + ; internally 
white. British Ocean. PI. vi. Fig. 51. 
5th Family. 
Concuz }. (7 Genera.) 
Three cardinal teeth at least on one valve ; as many, or fewer, 
on the other. Sometimes lateral teeth. 
The conchz constitute one of the most numerous and beauti- 
ful families of the conchifera, Their shells are equivalve, orbi- 
cular, or transverse, always regular, free, and in general very 
close, especially at the sides. They are more or less inequi- 
lateral, and seldom have true radiating ribs externally. 
The animal of the conche often projects beyond the shell 
two tubes, or siphons, formed by the mantle, one to convey 
water to the branchize and mouth, the other for its excretions, 
Its foot is remarkably lamellar. 
This family is subdivided into fresh water conche and marine 
conche: the first subdivision contains three genera, the second 
four, 
C. Fruyraties §. (3 Genera.) 
Shells covered with a false epidermis, and having two lateral 
teeth near the hinge. 
Like the naiada these shells are covered with a kind of 
greenish epidermis, which turns more or less brown, and is 
often eroded at the beaks. The fresh-water conchz inhabit 
lakes and rivers, keeping generally in the mud, with their beaks 
downwards, and buried in it, They are distinguished from the 
naiada by the projecting siphons of’the animal, and by the 
hinge of the shell, which has cardinal teeth like those of the 
* Devonshire. 
t Scalariformibus, arranged one above another, like a flight of steps, 
¢ Strictly bivalve shells, § Belonging to rivers 
