310 Lamarck’s Genera of Shells. 
veneres, whilst nothing similar is seen in the animal or shell of 
the naiada. The fresh water conche differ from the marine, 
not only by their localities, but by their having lateral teeth 
near the hinge, which are never found in the latter. 
1. Cyclas*. 
Shell ovate-globular, transverse, equivalve, beaks prominent. 
Cardinal teeth very small, sometimes scarcely any ; occasionally 
two on each valve, one of them folded into a double tooth; 
sometimes only one folded, or lobed tooth on one valve, and 
two on the other. Lateral teeth transversely elongated, com- 
pressed, lamellar. Ligament external. 
The cyclades are small shells, with thin valves, and never 
have three teeth on either. The beaks are never eroded. Some 
are so thin as to be transparent, and very brittle. They are 
grayish green, or slightly yellowish; some smooth, others 
transversely striated, with sometimes light-coloured bands. 
Type. Cyclas rivicolat. (Cyclas corneat. Draparn.) 
Shell subglobular, rather solid, elegantly striated, horn- 
green colour; internally bluish; two or three slightly coloured 
transverse furrows. Rivers of Europe. 11 Species. PI. vi. 
Fig. 52. 
2. Cyrena. 
Shell rounded-triangular, turgid or ventricose, solid, inequi- 
lateral, covered with an epidermis, beaks eroded. Three car- 
dinal teeth on each valve ; almost always two lateral teeth, one 
of them often situated near the cardinal. Ligament external, 
on the largest side. 
The cyrenz are generally thick, and rather large shells, al- 
ways covered with a greenish or brown epidermis. They are 
distinguished from the cyclades by having three cardinal teeth 
on each valve. They have also lateral teeth, one of which is 
often placed below the corselet. They inhabit rivers, and 
appear to be foreign to Europe. 
* Kuxdas, orbicular, having a rounded form. 
4 Inhabiting rivers. 
t The cyclas cornea (horny) of Lamarck (his second species) is the tellina 
eornea of Linnzeus. 
