248 
CONCHIFEEA. 
§ 5. Foot very small^ rudimentary^ byssiferous ; 
hinder adductor muscle large, placed for- 
ward in the centre of the lower edge of 
the shell; pedal opening small in front, 
near the umbo ; anal opening apparently 
behind and above the adductor muscle. 
Order II. Pholadacea. — Mantle with two close 
more or less elongated siphonal openings under j 
the hinder adductor muscle ; gills two pair, pro- i 
duced into the inhalant or lower siphon ; pedal 
opening generally small ; siphons united. 
Sub-order I. Orthoconchse. — Body symmetri- 
cal ; valves equal, gaping at each end ; man- ' 
tie partly exposed ; cartilage external or none. 
Living perpendicularly in holes in rock or 
ground. 
Sub-order II. Heteroconchse. — Body not sym- 
metrical ; valves unequal ; cartilage internal ^ 
in a pit ; hinge simple ; pedal opening inferior. 
Living lying on one side, in sand or mud. 
Sub-class II. Mantle lobes mostly free^ bearded be-- 
hind or on the whole edge^ sometimes with a 
separate siphonal opening for the vent: Asi- 
phonophora. 
Order III. Lasiacea, — Mantle lobes united, with 
an anal aperture under the hinder adductor 
muscle, and a pedal aperture. 
3. Pisidiadce. (p. 263). 
