186 ‘TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
perpendicular to the margin (fig. 29). These apparent 
prisms are, of course, the obliquely cut edges of the 
lamine. 
The shell is everywhere penetrated by very fine tubes 
and irregular channels. These are more apparent in the 
vertical section taken perpendicular to the shell margin, 
where they seem to be cut, for the most part, transversely. 
In a section at right angles to this they are by no means 
so obvious. It is evident from this that the black appear- 
ance of these cavities is due not so much to pigment, as 
Ehrenbaum supposes, as to the air included in the process 
of preparation of the section. They are very regular, 
following the planes of deposition of the shell lamine. 
The periostracum (Hpic., fig. 29) appears in section, not 
as a regular layer on the external surface, but rather in 
irregular fragments and patches. 
GENERAL ORGANISATION, MANTLE AND Foot. 
In the ordinary cockle not preserved by any special 
method, the animal is completely retracted within the 
shell. The shell margins fit together very closely except 
at the posterior extremity where, in the relaxed condition, 
the siphons protrude. Even here on account of the con- 
traction of the siphonal tubes, the mantle cavity is 
completely closed. In this condition it is difficult or 
impossible to open the shell without injury to the soft 
parts, and, when this is done, the animal is so much 
distorted owing to muscular contraction that the true 
relations of the parts are not evident. The animal is best 
prepared for dissection either by gradually adding a 4% 
solution of cocaine to the water in which the expanded 
animal lies, or by placing it at once in a 1% solution, and 
allowing it to extend, which generally happens in the 
course of an hour. The irritability of the siphons and 
