374 VENERACEA. 



single on each side, deeply plaited, united behind, and pro- 

 longed in front between the palpi, and each gill is tra- 

 versed by an oblique furrow. One species only is known, 

 from New South Wales. 



Order VENERACEA. 



Mantle provided behind with two more or less elongated 

 siphons, a lower or branchial, and an upper or anal ; siphons 

 usually separate, situated under the hinder adductor muscle ; 

 gills short, not produced into the lower or inhalent siphon. 

 Foot usually compressed, adapted for crawling and leaping. 



The families comprised in this division usually have the 

 foot laterally compressed and tongue-shaped for crawling, 

 but in the Cockles it is angularly bent for leaping, and in 

 the Chainas and Clams {Tridacnidce) it is small and rudi- 

 mentary, as these animals are fixed and stationary. In 

 some of the groups the siphons are long and more 

 or less separated at their extremities, in which case the 

 siphonal inflection is deep, as in the Venerida, Madrid, 

 and Tellinida ?; in others, the siphons are very short, and 

 the pallial line is consequently simple, as in the CardiidiC, 

 Chamidce, Tridacnidce, and Cyrenidm. 



Fam. MACTRID^. 



Labial tentacles long and pointed, pectinated on their 

 inner sides. Mantle-lobes more or less free beneath, united 

 before and behind, the margins more or less distinctly 

 fringed; siphons united to their extremities, which are 

 surrounded by fringes of simple cirrhi. Foot lanceolate, 

 sub-anterior. 



