BIVAL VES. 
409 
MARGIN OF MANTLE OF PECTEN, SHOWING TENTACLES 
and eyes (somewhat magnified). 
be found. They are irregular in their growth, and consequently the deter¬ 
mination of the species is a matter of difficulty. The pectens or scallop-shells 
(. Pectinidca ) are remarkable for the variety and beauty of their coloration and 
sculpture. I 11 most species the shells are nearly equivalve; but in a few, of 
which the common edible scollop is an example, the right valve is convex, and 
the left flat. One species (Pecten 
jacobceus) of the Mediterranean 
was worn as a badge by pilgrims 
who had been to the Holy Land. 
Most of the pectens are orna¬ 
mented with radiating ribs, but a 
few are smooth. Some swim 
freely by flapping their valves, 
others live permanently attached 
by a byssus. The animal has 
the mantle free, and frequently bears a row of brightly coloured eyes on the 
margin. The foot is small, the gills are extremely delicate, and the single adductor 
muscle in the adult is excentric. They are generally hermaphrodite, but sometimes 
the sexes are separate. 
More than a hundred 
species from all parts 
of the world and all 
depths have been 
described. Ten occur 
on the British coasts, 
and fossil species are 
numerous in all forma¬ 
tions, from the Carboni¬ 
ferous. The file-shells 
( Limidce ) somewhat 
resemble the scallops, 
but are nearly always 
white, and the edges of 
the mantle, which have 
no eyes, are furnished 
with lone; tentacular 
filaments. Some swim 
freely by flapping their 
valves, others attach 
themselves by a byssus, 
or, as in the accompany¬ 
ing figure, construct a 
nest of broken shells, 
stones, and other substances held together by a network of byssal threads. 
The two largest species occur off the coasts of Norway and Japan. The recent 
species of the chief genus (Lima) are not numerous, and some occur at 
file-shell (Lima) in its nest (nat. size). 
