1888 .] 
Mr D. M' Alpine on Bivalve Molluscs. 
179 
The same specimen was again carefully spread out, with the 
body-axis in the centre, to see if rotatory might be converted into 
translatory movement, but it commenced rotating at once right- 
handed, and the axis was soon visibly inclined to the left side. 
Although but 8 minutes elapsed from the last-mentioned quarter 
round, it was remarkable how the speed was suddenly increased 
when the parts were properly outspread. The last quarter round 
took 2 hours, and, starting anew, 8 minutes ; afterwards a quarter 
round was performed in 5 minutes, and the next in 3, the complete 
round being done in 23 minutes. Next day a quarter round was 
observed, the rotation still being right-handed, and it took 2 hours 
27 minutes, this being the last recorded. 
As regards translatory movement, it was only by accident, as it 
were, that it could be measured for any distance, since rotation 
might begin at once or be delayed for a little, and I did not use any 
guides to compel it to move straight forward. In one case inch 
was covered in 7 hours, and in another \ inch in 3 minutes, and 
I inch in 6 minutes, showing how variable the forward movement 
was when considered apart from rotation. 
Duration . — From the detachment of this specimen up to the last 
quarter was 37|- hours, and thus it had continued to move longer 
than the first specimen ; but as it only rested finally after doing 
about half a quarter round more, and as it moved very slightly after 
being noted 13 hours afterwards, we can say that it retained the 
power of motion for at least 50 J hours. 
The movements of detached parts will now be considered. 
Labial Palps — General Description. — The labial palps are two 
pair of triangular bodies of a deep flesh colour, one pair on each 
side of the mouth, attached dorsally by their broad base, and free 
at their pointed ends. The outer or anterior margin is more or 
less convex, while the inner or posterior margin is a little concave 
or almost straight. The inner and outer pairs will be considered 
separately. The two apposed faces of the outer and inner palps 
have a ridge running lengthways down their centre, with close-set 
transverse stripes proceeding from it towards the outer convex 
margin, while the other face of each is comparatively smooth. The 
outer is the stouter of the two, capable of more prolonged exertion, 
and of expelling larger masses from the body (fig. 2, a, h). 
