BIVALVIA. 97 
Unto, Retzius, 1788. 
Mya (sp.), Zinn., 1747. Synroxta. Rafinesque, 1820, 
TriauEeTRA. Klein, 1753.* Atasmoponta. Say, 1820. 
Livy ©, and LimyZopeRMA (sp.). Poli., 1791. Mysca. Turton, 1822. 
CrisTaria. Schum., 1817. Dretopon. Spiz, 1827. 
PAxYODON. Id. a TrerrRaPLopon. Id. ,, 
PRISODON. Id. Fp, Lasmicona? Rafin., 1831. 
Mareanritana. Id. % LasMonos? Id. 35 
AMBLEMA. Rafinesque, 1819. Monoconpyima. D’Orb., 1835. 
ELLIPTIO. Td. 33 ARGLIA. Swainson, 1840. 
Eurynia. Id. » Cantuyria. Id. Ee 
OBOVARIA. Id. a3 CALCEOLA. Id. (not Lam.), 1840. 
PLAGIOLA. Id. és CompLanartia. Id. 1840. 
PLEUROBEMA. Id. 38 CuntIcvLa. Id. 5 
PROPTERA. Id. > HeEmIopon. Id. = 
TRUNCILLA. Id. 55 Hyriperta. Td. > 
AXIMEDIA. Id. 1820. TRIpEA. Id. Pe 
DIPLASMA. Id. re LicuMIA. Id. 52 
ELLIPSARIA. Td. 55 LYMNADEA. Id. SS 
LamPsILts. Id. 53 Mecapomes. Id. “A 
LEPTODEA. Td. a Nata. Id. 3 
METAPTERA. Id. or NaIpeEa. Id. 3 
OBLIQUARIA. Td. = PorTaMIDA. Id. 5 
QUADRULA. Id. - THELIDERMA. Id. a 
RovruNnDARIA. Id. or UNIopstIs. Id. 35 
ScCALENARIA. Id. AS Luricoua. Goldfuss, 1846. 
Generic Character. Shell equivalve, inequilateral, generally thick and externally 
smooth, occasionally ornamented with nodules or spines. In the recent state covered 
with an epidermis, often wanting at the umbones, where the shell is sometimes much 
eroded. Hinge, with two cardinal teeth in the left valve, and one, sometimes two, in 
the right, an elongated lateral tooth on the posterior side. Impressions by the 
adductor muscles large and deep, that by the mantle without a sinus. Ligament 
external. Shell nacreous. 
Animal of the form of the shell, its mantle open in front, with simple edges or 
slightly fringed; siphonal tubes short, plain or fringed, sometimes scarcely defined ; 
foot large, compressed. 
This is pre-eminently a fresh-water genus, and although found living in the rivers 
of Europe, Asia, and Africa, appears to be only fully developed in the lakes and 
rivers of North America, more than 200 species have been named and described from 
that part of the world. 
* In compliance with the recommendation of the Committee appointed by the British Association for 
the Advancement of Science, 1842, ‘‘to consider the rules by which the Nomenclature of Zoology may be 
established on a uniform and permanent basis,’’"—the 12th edition of Linnzeus’s ‘Systema Nature,’ 1767, 
is made the starting point from which the dates of priority have been adopted. 
13 
