CONCHIFERA. 



247 



CLASS II. CONCHIFERA Lam. 



THE animals without any distinct head, the mouth 

 being situate between the four leaf-like lips ; with 

 a more or less distinct, compressed, central foot, 

 enveloped by the two pair of leaf-like gills and the 

 large leaf-like mantle, which are covered by two 

 shelly valves united together by a ligament along 

 their dorsal edge. 



This class is divided into sub-classes, orders, and 

 sub-orders, thus : 



Sub-class I. Mantle leaves connected together with 

 two siphonal openings behind : Siphonophora. 



Order I. VENERACEA. Mantle with two more or 

 less elongated siphonal openings ; gills short, 

 not produced into the inhalant or lower siphon ; 

 siphons often more or less separate under the 

 hinder adductor muscle. 

 1. Foot compressed ; animal crawling. 



1. Cyclada. (p. 250.) 



2. Foot conical, acute, angularly bent behind 



for leaping. 

 3. Foot truncated and dilated at the end for 



crawling and anchoring. 

 4. Foot elongate, slender, strap-shaped, byssi- 



ferous for anchoring. 



2. Dreissenada. (p. 258.) 



R 4 



