58 EOCENE MOLLUSCA. 
DREISSENA. Van Beneden, 1834. 
Mytitus. Lam. and Goldf. Conceria. Partsch, 1837. 
Enocepnatus. Minst., 1831. Mytitomya. Cantraine, 1837. 
Mytuttna. Cantraine, 1834. Mytinimerta. Conrad, 1837. 
DytHaLMIA. Jay, sec. Gray. CorLoconta. Bronn (laps. calam.), 1837. 
TrcHoGoNtA. Rossmaesler, 1835. | Aixocepuatus. Herrm., 1846. 
Generic Character. Shell equivalve, inequilateral, ovately trigonal or obscurely wedge- 
shaped ; umbones terminal, pointed ; hinge edentulous ; valves obtusely keeled, with a 
slight byssal sinus ; one muscular impression, immediately beneath the umbones, supported 
upon a shelf, projecting inwardly ; impression of pedal muscle single; anal adductor 
large, excentric ; connexus bipartite. 
The animal (AZytlus polymorphus) with the mantle closed, except for the foot and 
byssus. Siphons unequal; anal opening small, conical, and plain; branchial opening 
prominent and fringed ; foot-muscle short and thick. 
The name of Hnocephalus of Minster seems to have priority of date, but, according to 
M. Bronn, it was merely given to the shell without figure or description; it therefore 
yields to a subsequent claim, and this is divided between Mytilina, Cantraine, and Dreissena, 
Van Beneden; both of which names were proposed for the shell in the same year. Bronn 
has awarded the right to Van Beneden, who, he says, is entitled to priority. The name is 
also in general use. 
This is a fresh-water mussel; the typical species, M/yt. Voly@, Chemn., is a native of 
the Aralo-Caspian rivers, whence it has been brought to this country on foreign timber, 
and it has now become naturalised with us. It differs principally from AZy/i/us in having 
a calcareous plate immediately under the umbo ; in other respects it resembles the general 
form of the mussels. M. Nyst enumerates fourteen species, eight of which are fossil 
from the Tertiaries of Germany, France, Belgium, and the Crimea; but a fossil said to 
belong to this genus has been found in the coal measures. ‘The species, though probably 
more inclined to fresh-water, were capable of living in estuaries where the water was salt. 
This, like the genus Unio, is very variable in regard to the solidity or substance of the 
shells of different species ; some are thick and heavy, while others are quite the reverse. 
This is, perhaps, truly Dimyarian, with two adductor muscles, and the animals possess 
distinct but short siphonal tubes, indicating, by the inflexion in the mantle-mark, probably 
a slight difference in their extent of protrusion.’ 
1 Dreissena is said to be distinguished from Mytilus as much by the intimate structure of the shell 
as it is by other characters. 
