CONCHIFEEA. 
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. Cycladce. (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 anchorino*. 
o 
2. Dreissenadce. (p. 258.) 
