39 



also other parts secrete the two rairs of knobs on the 

 valves. The posterior edr^e of the shell is not marked by 

 the tliickened mantle ea^e at; in otiier types, but lorms a 

 duplicature around the v/hole posterior edge of the shell, 

 which stretches for^^fards. In the dorsal ret^ion, the whole 

 umbonal region of the valves is covered by this duplica- 

 ture and tO- this special part, Quatrefages gave the name 

 of "cephalic-hood". (£.h. figs. 7,28). To it he assigned 

 the fvmction of fomning the burrow. It is somewhat, but 

 not very muscular, and no such important work could be 

 done by it. In the collar region it has been seen that 

 the mantle forms Lho duplicature or collar which projects 

 posteriorly over the base of the siphons; and within the 

 cavity of which the handles and paddles of the palettes 

 are formed separately (figs. 7,10). The siphons as part of 

 the mantle, form tvro long tubes (figs. 7-9) which are fus- 

 ed together through half or more of their extent. The 

 anal or exhalent siphon is wit;:out papillae or tentacles 

 and is shorter and less muscular than the respiratory or 

 inlialent siphon, which bears a number of tentacles (figs. 

 7-9), These are sensory structures, which also serve to 

 close the entrance of the respiratory siphon very quick- 

 ly. 



