CONCHIPEEA. 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 toitJi 

 tioo 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. Cycladce. (p. 250.) 



§ 2. Foot conical, acute, angularly bent behind 



for leaping. 

 § 3. Foot truncated and dilated at the end for 



crawlino; and anchorino;. 

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



ferous for anchoring. 



2. DreissenadcB. (p. 258.) 



R 4 



