FIJI-NEW ZEALAND EXPEDITION 73 
tion arises whether these two masses might not represent both 
posterior and anterior adductors. 
The mantle itself is, as already indicated, double edged; the 
margin nearest the shell is very thin and does not project as far 
as does the inner border which comes to the very edge of the 
valves, is very muscular and rugose on its exposed surface where 
there are also numerous rounded colored tubercles which we took 
to be eyes like those of the scallop or Pecten. The color is com- 
pletely destroyed by formalin in the case of Tridacna, while a 
good deal of it remains in Pecten. On the inside, towards the 
median line of the body, this muscular part of the mantle sends 
off a much thinner sheet of tissue which meets a similar one from 
the opposite mantle coalescing with it along the mid-line so that 
the whole structure is continuous, except for two rather small, 
slit-like apertures toward the ends of the gaping shells. 
The foot, instead of projecting ventrally as in most bivalves, is 
turned abruptly towards the dorsum so that the large byssus ex- 
tends through the great gaping dorsal aperture of the shell. This 
form being fixed, the foot has no function in locomotion but is 
highly muscular so as to form a firm anchorage for the tuft of 
fibers that make up the byssus. The byssal mass seems capable of 
partial retraction into the foot which accounts for the habit of 
this clam of shrinking tightly against the rock when touched, so 
that one can get no purchase with his fingers if he attempts to 
tear it from its anchorage. The excessively numerous byssal fibers 
form a compact oval mass of parallel threads about an inch long; 
each thread is terminated by a flattened cap which adheres strong- 
ly to the rock, and the whole mass, deeply embedded in the eylin- 
arical foot, ends in the extensive byssal glands. 
Taking it all in all, the extraordinarily developed byssus is one 
of the outstanding features of Tridacna. The portion of the man- 
tle immediately surrounding the byssus and lying between that 
organ and the opening in the shell is strongly papillose; some of 
the papille branch. The appearance is that of a glandular organ. 
The labial palpi are poorly developed and hardly distinguishable. 
An almost black species of Iithodomus was found in holes ap- 
parently bored by it in the reef rock. It is almost cylindrical in 
shape, the umbonal region is smooth, but the rest of the shell, ex- 
cept the extreme posterior end, is very finely crenulated with 
vertical markings that extend across the lines of growth. 
Among the other lamellibranchs were a fine Pecten, a few small 
