PEARL SHELLS. 249 
many of them may every day be seen at this 
work.” 
GENUS PINNA. 
The largest British bivalves belong to the genus 
Pinna, in which the shell is fibrous, and horny in 
texture, rather fragile and delicate ; the valves are 
equal, triangular and exceedingly one-sided; the 
hinge is straight, long and toothless, the ligament 
marginal, and almost wholly internal. 
The animal is triangular, with the mantle freely 
open and having fringed edges ; the foot is small, 
conical, or tongue-like, and grooved; there is a 
small anterior muscle in the angle; the posterior 
one is by far the larger. 
The genus contains but few species, which are, 
however, widely distributed, and most of them are 
of large size; they range from deep water to near 
the shore, living on sandy or muddy bottoms, with 
their beaks plunged deep into the ground, and the 
broad extremity gaping upwards. 
In a cabinet of British shells the eye is sure 
to be attracted by the great fan-like horny valves 
of the Pinna (Pinna pectinata), which are not un- 
commonly seen twelve inches long, and seven broad 
at the gaping end. Young specimens are of a semi- 
transparent horny texture, partially covered with 
rows of hooked spinous scales; but these spines are 
almost always worn away in old examples. The 
interior is pearly in a slight degree, and loose pearls 
of dull colour are sometimes found within the shells. 
The silky byssus spun by these mollusks, which 
is capable of being woven into small articles of 
wearing apparel, has been already noticed. 
The great Pinna is found all around the British 
