LAMELLIBRANCHIA. 33 
The MytiUdce, or Mussels, are too well known to need descrip- Cases 142- 
tion. The small foot, which is brown in the common species, is 145 - 
not much used in creeping about, but has Fig 27 * 
the power of spinning a byssus or bundle 
of tough threads, by means of which the 
animals attach themselves to rocks and one 
another, forming colonies of vast numbers. 
Mussels have always been much eaten in 
this and other maritime countries, and large 
quantities are brought to the London market 
from the Dutch coast. At times they are 
unwholesome ; but all the exact causes of 
this are not known. Mussels seem to be 
found on every shore, and some of the Trigonia margaritacea. 
species are very widely distributed — the Case 141. 
common edible Mussel, M. edulis, being found on every European 
coast, on the shores of North and South America, in the Arctic 
and Antarctic Oceans, and probably on the coasts of Australia. 
One group of Mussels (Lithodomus, Case 144) burrow in rocks and 
other shells, forming holes just large enough to contain their shells. 
L. dactylus is sold as an article of diet on the shores of the 
Mediterranean. 
PSEUDOLAMELLIBRANCHIA. 
The gills in this order are plicate, and the two lamellae of each Cases 145- 
plate are furnished with conjunctive or vascular interlamellar junc- 1 60 * 
tions, and the filaments are connected by internlamentar ciliated discs 
in some cases, in others by vascular concrescence. The mantle- 
margins are separated all round, and the foot is either small or 
wanting. Only a single adductor muscle is generally present. The 
Pearl-oysters, true Oysters, and Scallops are the forms which consti- 
tute this order. 
The large family of Aviculidce includes the "Wing-shells" 
(Avicula), the " Pearl " and " Hammer Oysters " {Meleagrina and 
Malleus), and Pinna (Fig. 28). Some species of Pinna attain to a 
length of two feet. They are found imbedded in the sand with the 
narrow pointed end downwards. They form a large silky byssus, 
which can be woven or knitted into gloves, socks, etc. (see side table- 
case B). The " Hammer-Oyster " {Malleus) is so called from its rude 
* From ' The Cambridge Natural History.' Messrs. Macmillan & Co. 
D 
