358 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
be seen, appears as an appendage quite distinct from the 
rest of the visceral mass, and contains no extensions of the 
reproductive organs. 
The pericardium (fig. 1, Per.) is situated posterior to 
the digestive gland. The rectum (fig. Ad. c. 5) passes 
through the ventricle of the heart, which is enclosed by 
the pericardium, and is continued over the adductor 
muscle, to which it 1s attached, bending to one side of the 
median line and eventually terminating in a lipped anus. 
The aperture of the mouth is placed not far above the 
foot on the anterior surface of the digestive gland between 
the two very conspicuous dendritic lips, pigmented with 
an orange colour (fig. 1, L. p.). 
At each side where the gills terminate dorsally are 
two flaps, also pigmented with a yellowish brown colour. 
These are the Labial Palps (fig. 1, L. p. e.); they become 
continuous dorsally with the lips. 
The renal organs (fig. 1, A. 0.) are situated on each 
side of the reproductive portion of the viscera between it 
and the gills, and the external opening at their ventral 
end serves both as renal and reproductive aperture (fig. 1, 
Ro. rp.). 
The positions of these various organs in relation to 
the shell are not the same as those in the Dimyaria. Thus 
the pericardium is posterior, the digestive gland (“ liver ’’) 
is dorsal and the foot and visceral mass are situated 
anteriorly, the hinge line beine considered as dorsal. 
Owing to this, some authors have regarded the hinge 
line as dorso-anterior, and the antero-posterior diameter 
as represented by a line drawn from the front corner of the 
hinge line to the point where the rectum ends. The 
position of the organs is regarded as due to an increase in 
size of the posterior adductor after the disappearance of 
the anterior adductor, together with a movement of the 
