412 
bulletin of the bureau of fisheries. 
Even in the present day the fiber is utilized, but more for its rarity than anything else. The women 
comb the lana fbyssus] with very delicate cards, spin it, and make from it articles which are much 
esteemed for the suppleness of the fiber and their brilliant burnished gold luster. 
A considerable manufactory is established at Palermo; the fabrics made are extremely elegant 
and vie in appearance with the finest silk. The best products of this material are, however, said to be 
made in the Orphan Hospital of St. Philomel, at Lucca. 
This byssus forms an important article of commerce among the Sicilians, for which purpose con- 
siderable numbers of Pinna are annually fished up in the Mediterranean from the depth of 20 to 30 feet. 
Under normal conditions Atrina occupies one position during its entire life — 
nearly buried in the mud, with its anterior end downward. The enormous byssus 
extends deep into the mud and attaches to shells and coarse pebbles. Specimens are 
most easily collected in calm weather at low tide, when they can be seen extending an 
inch or less above the surface of the mud. 
* In the following discussion, although the continuity is thereby interrupted, it seems 
advisable to treat the organs under separate headings, passing briefly over those which 
have yielded nothing of particular interest. To avoid repetition the anatomy and 
physiology of the organs will be treated together. The general anatomy is shown in 
figures 16 and 20. 
Since every species is adapted to its peculiar mode of life certain anatomical features 
are better understood when their function is known. It has therefore been my pur- 
pose to study habits and function as well as anatomy. 
SHELL. 
The shell valves are large in comparison with the size of the body, and they are 
united to each other along one side by a hinge ligament which extends in a straight 
line from their anterior to their posterior ends. The hinge ligament is more or less 
calcified, so that it is not greatly different from the other parts of the shell. The outer 
surface of each shell is studied with spines, which are distributed in rows radiating 
from the anterior pointed end as a center to the posterior end. Primary, secondary, 
and tertiary rows of spines may be distinguished in the shell of a large specimen. The 
portion of the shell which lies posterior to the adductor a consists of a single layer in 
contrast to the typical lamellibranch shell, which has three layers, easily distinguishable 
by difference in structure or material. It apparently corresponds to the middle or pris- 
matic layer of the typical lamellibranch shell, being composed of prisms which lie at 
right angles to the surface. When the surface is examined with a compound micro- 
scope it appears honey-combed, while a transverse section, obtained by grinding, looks 
not unlike a lot of quartz crystals corded like wood. (See fig. 1.) It is possible to 
dissolve out the lime salts with acid, leaving behind only the organic matrix. This 
matrix resembles cork in many respects, but when examined histologically it is seen to 
have the same gross structure as the shell before treatment with acid, except that the 
chambers formed by the organic matrix are now empty. 
a I refer here to the posterior adductor muscle, and unless otherwise stated further references to the adductor may be taken 
to mean the posterior adductor. 
