248 CONCHIFERA. 



§ 5. Foot very small, rudimentary, bysslferous ; 

 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 Avith two close 

 more or less elongated siphonal openings under 

 the hinder adductor muscle ; gills two pair, pro- 

 duced into the inhalant or lower siphon ; pedal 

 opening generally small ; siphons united. 



Sub-order I. Orthoconchffi. — Body symmetri- 

 cal ; valves equal, gaping at each end ; man- 

 tle partly exposed ; cartilage external or none. 

 Living perpendicularly in holes in rock or 

 ground. 



Sub-order 11. Heteroconcha?. — 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 ivith a 

 separate siphonal opening for the vent : Asi- 

 phonophora. 



Order III. Lasiacea. — Mantle lobes united, v/ith 

 an anal aperture under the hinder adductor 

 muscle, and a pedal aperture. 

 3. Pisidiadce. (p. 263). 



