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shell, as a whole, is very thick and elastic and extends past the 
calcareous portion of the shell about one-fourth the entire width of 
the shell, Fig. 7. Furthermore, the cuticle is made up of altern- 
ating thick and thin portions that radiate from the beaks to its free 
margins. The thickened portions insure great elasticity and the 
thinner portions allow plaiting when the cuticle is folded. 
The mantle-lobes lining the shell-valves are united along their 
ventral borders, Fig. 8, and leave only a small posterior opening, po, 
through which water can be forced, and a larger anterior opening, ao, 
through which the foot can be protruded. 
Fig. 9. Fig. 10. 
Fig. 9. Preparation of the ventral borders of the mantle of Solenomya velum, 
showing the pallial muscles. ao anterior opening of the mantle chamber; po posterior 
Opening of the mantle chamber. 
Fig. 10. Transverse section of the ventral margins of the mantle and cuticle of 
Solenamya velum, here shown partially infolded into the mantle chamber. e cuticle of 
the shell; Zm longitudinal pallial muscle; rm bundles of radial pallial muscles; s the 
position occupied by the shell. 
Along the line where the margins of the mantle have united, and 
extending some distance on either side, is a strong longitudinal muscle, 
Fig. 9. Around the opening through which the foot can be protruded, 
ao, and around the smaller posterior opening, po, this muscle spreads 
out to form sphincters. 
The radiating pallial muscles are very strongly developed, Fig. 10 
ym, and extend from the calcareous portion of the shell to the free 
margin of the cuticle. These muscles do not extend continuously 
from the calcareous portion to the free margin of the cuticle, but 
they are broken up into shorter bundles, many of which are attached 
to the flexible cuticle at both ends. 
The foot is elliptical in cross section, and somewhat larger at its 
free end than it is nearer the body. 
