Corculum and Zooxanthellae — KAWAGUTI 
45 
Fig. 4. Reconstruction of a series of growth forms of C. cardissa. (Explanation in text.) 
The shell is remarkably thin, as is shown 
in Table 1. It is also remarkable in the uni- 
fortuity of the thickness throughout the whole 
shell Only at the anterior, that is, lower, side 
the ventral portion shows a slight thicken- 
ing. In a very small specimen the shell is 
very thin but it rapidly attains a maximum 
thickness of between 0.5 and 0.7 mm. The 
area of cross section of shell is measured and 
weight of shell per unit of this area is cal 
