368 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
surface with the byssal groove faces the right valve, and 
it will be remembered that it is the right valve of the 
shell that has the indentation known as the byssal notch. 
Though there are no traces of the byssus in the adult 
P. maaimus, the byssus gland is very well developed. It 
is situated very deeply in the tissue, in fact practically 
outside the foot, in the midst of the retractor muscle. If 
a series of transverse sections is followed from the byssus 
groove region of the foot to the retractor muscle, the 
following sequence will be observed :——The byssal groove 
is rather wider at the bottom, and this cavity runs in 
towards the byssus gland. In sections taken below the 
byssus groove the sides of the groove have coalesced and 
the cavity alone is present. As we pass further in, the 
dorsal wall of the cavity becomes ridged by longitudinal 
projections, which gradually meet the ventral wall, so 
that ultimately the original cavity is divided up into 
compartments by parallel partitions running across from 
the dorsal to the ventral wall (fig. 47, B.g., and fig. 9). 
These compartments are deep and wide, but very narrow. 
Sections showing this structure pass through the retractor 
muscle alone, and are therefore posterior to the actual 
foot itself. 
The partitions are composed of connective tissue in 
which are to be found many muscle fibres, and are 
bounded by a layer of epithelial cells almost cubical in 
shape, and of course continuous with those of the byssal 
gland. They are well provided with cilia. The 
compartments terminate blindly, and at the same time 
become reduced in width; but at their blind ends, the 
cells (fig. 9 B.g.c.) are rather larger than the other 
epithelial cells and contain practically no contents. A 
structureless body, presumably the nucleus, is present in 
each, but it stains differently from the other nuclel and 
