372 Etheriid^.- MOLI 
six hundred thousand young shells. This genus is 
represented in Plate 11, fig. 13, by Anodonta cygnea. 
Family — ETHERIIDiE. 
The Ethcrias have the shell irregular in shape, in- 
equivalve, covered externally with a thick olive epider- 
mis, and pearly within. The outer surface of the valves 
is vei-y uneven, and frcquentlj' beset with tubular pro- 
cesses, while the internal surface is blistered, as if with 
air-bubbles, leaving cavities in the form of small vesi- 
cles, or very thin bladders between the plates. The 
hinge is w'ithout teeth, and the ligament subinternal. 
The adult shells are always attached to each other, or 
to foreign bodies, by the outer surface of one of the 
valves ; but when young is free, like Anodon. In this 
young state the animal, as Dr. Gray observes, may 
possess a foot, but being adherent when adult, this 
organ becomes unnecessary. The little cavities in the 
inner substance of the shell, are believed to be filled 
with water when the animals are alive. 
Genus Etheria. — This genus adheres by the beak. 
The species are all natives of fresh water, being chiefly 
found in the river Nile, above the Cataracts, and in 
the river Senegal. Bruce, in his “Travels,” mentions 
these shells under the name of “ fresh-water oysters.” 
They are collected, he informs us, by the natives as 
articles of food, and the shells are employed by them 
for ornamenting their tombs. For a representation of 
the family, see Plate 11, fig. 16 — Etheria caillaudii. 
Family— MYTILID^. 
The Mussels secrete a byssus. The shell is equi- 
valve, elongated, closed, with the beaks anterior. The 
valves are covered externally with a dark thick epi- 
dermis, and they are more or less pearly within. The 
hinge is without teeth, and the ligament is internal, 
submarginal, and very long. 
Genus Mytilus, the Sea Mussel . — This genus has a 
wedge-shaped shell, rounded behind, and the beaks ter- 
minal and pointed. The species of Mytili are numerous, 
and are world-wide in their distribution. The byssus 
which the animal spins is strong and coarse, and these 
shells are usually found attached to rocks, stones, float- 
ing bodies, &c. It appears, however, from the observa- 
tions of Dr. Gould of Boston, that mussels are not quite 
stationai’y animals, but that they can move from place 
to place with great facility. The manner in which the 
common edible mussel, Mytilus ed^dis., forms its con- 
nection with the rocks and stones, &c., is explained by 
Reaumur. By placing some individuals in a vase of 
sea- water, he was able to watch the process. “ Opening 
their valves, the foot was first protruded, and with 
various strains and stretches, gradually tbrnst out, 
until at length the elongation was carried to the de- 
sired extent, sometimes to fully two inches. It was 
now employed in feeling or testing all the objects 
within reach, directed to the right or left, or backwards 
or forwards. After all this prelude, to ascertain ap- 
parently the security of the intended holdings, the 
point of the foot is settled and retained for a short time 
on the chosen spot, when again, it is suddenly removed. 
^jUSCA. Lithodomus. 
and immediately withdrawn entirely within the shell, 
leaving behind a thread that reaches from the spot to 
the base of the foot. By many repetitions of this 
operation, carried on patiently day after day (for not 
above four or five threads are spun in the twenty-four 
hours), and by attaching the disc-like extremities of 
the threads to different places, the mussel at last com- 
pletes its cable, and secures a safe anchorage.” 
Mussels are also used as bait for fishes, and as food 
for man. 
THE EDIBLE MUSSEL {M. edulis, Plate 1 1, fig. 17) 
frequents mud banks, which are uncovered at low water, 
are very prolific, and attain their full growth in a single 
year. The}" abound in a great many parts of our coasts, 
and immense quantities are yearly consumed. Their 
fishery occupies a considerable number of persons, espe- 
cially where the mussels have obtained a reputation for 
superior excellence. “ This is the case with the mus- 
sels of Buddie Bay, on the coast of Northumberland ; 
and those of Isigny, near Bayeux, and of other places 
on the western coast of France, are held in high esti- 
mation. Mussels, however, of inferior delicacy are 
found in very great abundance on tbe rocks which 
border the coast between St. Malo and Cancale, in the 
department De la Manche, where they are torn from 
their attachments by means of an iron hook at low 
water ; and the annual profit of this fishery is estimated 
at from 2000 to 2500 francs.” — {Johnston.) The con- 
sumption of mussels in Edinburgh and Leith is estimated 
at four hundred bushels (= four hundred thousand 
mussels) annually; enormous quantities, Mr. Woodward 
informs us on the authority of Dr. Knapp, are also used 
for bait, especially in the deep-sea fishery, for which 
purpose thirty or forty millions are collected yearly in 
the Firth of Forth alone. It is a curious fact that of 
all edible mollusks, the common mussel is that which 
proves most frequently poisonous to those who eat 
them. Dr. Johnston says that he has known them to 
produce an itchy eruption and swelling over the whole 
body, attended with great anxiety and considerable 
fever. Many instances of their having proved dele- 
terious are mentioned in this author’s excellent “ Intro- 
duction to Conchology,” and amongst others one that 
occurred in 1827 in Leith, which I well remember as 
having produced a great sensation in that town and 
in Edinburgh. Two or three deaths took place, and 
upwards of thirty people were severely affected. The 
cause of this unwholesomeness of mussels, at certain 
seasons, is still a matter of conjecture. 
Genus Modiola or Horse-Mussel. — This genus 
has an oblong shell, inflated in front, and the beaks 
obtuse and placed a little behind the extremity The 
species of Modiola are numerous — about seventy hav-. 
ing been enumerated. They are chiefly tropical in 
their distribution, though a few are found in the Medi- 
terranean, Great Britain, and even in the Arctic seas. 
They differ chiefly from the Mytili in the position of 
the beaks, and in their habit of burrowing cr spinning 
a kind of nest in which they occasionally conceal them-' 
selves. They range from low water to an hundred 
fathoms. 
Genus Lithodomus. — The Date shells have a^ 
cylindrically oblong shell, rounded and inflated in front, 
