A60 MOLLUSCA. 
to great length, carrying a byssal thread, and attaching it to the object 
towards which it is moving; by a sudden contraction, the shell is jerked 
forwards the length of the thread ; and this operation is repeated until 
the desired spot is reached, when it is there anchored by a number of 
threads thrown out in various directions. ‘These threads are very 
elastic, and are found to proceed from the cavity at the base of the 
foot. 
On some specimens the branchie were found to have the intervals 
of the strie filled with minute, but perfectly formed shells, those 
nearest the margin being dark-brown, while those towards the dorsum 
were paler and nearly white.  [s. p. c.] 
It will be perceived that the animal of this shell varies from that of 
Mytilus, in having the mantle closed, except at three orifices. On the 
other hand, their interior structure, and especially the apparatus and 
manner of locomotion, correspond precisely. As the animal differs 
from that of Mytilus in characters which are considered of generic 
value, it is proposed to give the name of Gaimarptia, after M. Gaimard, 
who, in all probability, was the first to observe it as an object of scien- 
tific interest, and who has also done so much else for science. 
Figure 568, lateral view of the shell and animal; 568 a, dorsal view 
in outline; 5684, ventral margin, showing the opening of the mouth; 
568, the gills; 568 d, the viscera; 563 e, two views of the foot; 568 f, 
mouth and palpi. 
SHELL ovate-trapezoidal, thin and fragile, ventricose, covered by a 
smooth, shining epidermis, of a pale olive-colour usually, but occa- 
sionally bright yellow, orange, dark brown, and almost black; beaks 
near the anterior end, prominent, antrorse, contiguous; anterior end 
compressed, sloping abruptly to the base; dorsal margin nearly recti- 
linear; posterior margin regularly rounded; ventral margin somewhat 
sinuous and gaping anteriorly, forming a small elliptical passage for 
the foot and byssus; the right valve has two small, oblique, cardinal 
teeth, which receive two corresponding ones of the opposite valve ; 
muscular impressions quite distinct, united by a somewhat sigmoid 
pallialimpression. Substance of the shell purplish or chestnut-coloured 
externally; yellowish towards the margin; interior always deep purple 
within the pallial impression, and rosy white outside of it. 
